Sorry for being unclear. English is not my first language. I will try to be more clear and thorough this time.
Your English is fine. I wasn't sure what you meant.
Wouldn't it make sense that the exercise has to be different enough? For example, in the bench press, if I move just one finger width wider or narrower, the difference, or variety, will not be as optimal as if I switched from close grip to a wide grip? Now, if this is so, on what basis would we make the distinction? Distance? Difference in strength levels? Difference in primary muscles used in the movement? And at what point, if we exacerbate the difference, will the exercise become too different, so that the carryover to the main exercise will be diminished?
Making The Exercise Different
Moving your bench press grip one finger wider isn't going to make it different enough.
It's like making a one shade of the color red minutely a shade lighter. Both shades of the color red looking like each other. Making one a dark red and the other a light red make the difference much more distinct.
In the case of the bench press, you need to change your grip so that it is noticeable different. Something you can see and feel. You need to play with it.
Difference In Strength Levels
There is going to be some strength variances when you change you grip width or change your pressing angle. You are going be stronger in a Decline Press than in a Flat Bench Press and Stronger in a Flat Bench Press than an Incline Press.
That means each type of pressing will have a different 1 Repetition Max that you need to base your percentages on.
Difference In Prime Movers
Changing the angle of a movement, modifies how the prime mover work.
As an example, a Conventional Deadlift engages the lower back to a greater degree than the Sumo Deadlift.
Research (Dr Tom McLaughlin, PhD-Biomechanics) determined that the lower back initiates driving the bar off the floor in a Conventional Deadlift, with some assistance from the legs.
The Sumo Deadlift drives that weight off the floor with the legs, with some assistance from the lower back.
Performing exercise that are similar in nature to let's say the Conventional Deadlift will have more carry over. However, performing Sumo Deadlift also will assist in increasing strength in a Conventional Deadlift.
Second, regarding variety through exercise selection vs. variety through load:
As a premise I would separate strength training to the maximum effort method vs the repetition method and the submaximal method, roughly categorizing them as ME vs. RE/SE.
Conjugate Training
The anecdotal and research data prove that combing different type of strength promotes an increases in strength and size, dependent on the emphasis of the program.
Olympic Lifter focus on strength and power exercises. Chris Thibaudeau wrote an article on how the Chinese Olympic Lifter are employing more Bodybuilding/Hypertrophy Training.
The Westside Powerlifting Method does the same with "Speed Days (Power Training), Repetitive Method Days (Hypertrophy Training) and Max Effort Days (Limit Strength).
Dr Michael Zourdos' (Powerlifter) research determined greater strength was obtain when a program included: Hypertrophy/Bodybuilding, Power and Limit Strength.
There are various way of writing it into a training program.
Now, based on your understanding, what kind of difference in seeking variety from exercise selection vs load is there between the methods? Is variety through exercise selection more important for ME than RE/SE? How about variety through load? Etc.
Auxiliary Exercises
One of the most effective method for increasing strength is to vary your Auxiliary Exercises, as the Westside Method does.
Perform one Auxiliary Exercise in a Training Cycle, then replace it with another exercise in the next Training Cycle.
Varying Load
With the exercise you select, you want to at some point in the Training Cycle, push the limit of the load it.
Your volume to a large extent will determine the load.
As an example, in performing one all out set of 5 repetition in an exercise, you are going to push a higher load than if you are performing 5 sets of 5 repetition in the same exercise.
Chris Thibaudeau's Neuro Training Type
Chris Thibaudea does a nice job of breaking down the different type of training protocols based on individuals in his podcast at the site above.
I posted the notes on each one of his podcast.
Speed and Power Training Percentages
Speed Training and Power are developed with different percentages of your 1 Repetition Max, as is Limit Strength.
For Speed and Power Development, I am a proponent of training their designated percentages with a variety of load; which change their percentages.
Let me use my Speed-Power Squat day for this example.
Speed in traditional movements is developed with load of 10-40%, with around 30% of 1 Repetition Max being the sweet spot.
In a traditional movement, power is optimized with load of 48 - 62% of your 1 Repetition Max.
Progressively increasing the load in each set of Squat Set engages the muscles and muscle fiber differently, provides variety.
My first two warm up sets in the the Speed-Power Squat involve Speed Training Percentages, under 40% of 1 Repetition Max. They are Speed Squats.
My third set is the entry into a Power Squat, with a load of around 48%.
My next two Power Squat push the weight up into the higher Power Percentages.
"Is variety through exercise selection more important..."
Exercise variety as well as is incorporating Limit Strength, Power, Speed and Hypertrophy is important.
Homeostasis
Your body is like an over protective Mommy. It works at keeping you stable so that you don't hurt yourself.
You have to play tricks so that you can get stronger, faster or bigger.
Varying your training exercises, volume, load, frequency, and combinations are some of the tricks you can use to make progress.
Kenny Croxdale