There has to be a convergence of variables, I suspect O2 depletion is the single most important, but I really couldn't say without a lot more digging.
Isometric MVC even when held for more than 30 sec only generates a modest increase in HR according to the literature (I intend to test this next time I do my Iso. Either BP or HR must be going through the ceiling based on RPE). A sprint either on foot or on a bike tends to spike HR to > 100% max within 10-15 seconds in most people.
The few times I used a monitor during heavy resistance training at a modest cadence my HR never went above 160 even at fully exhausted.
A max effort sprint is going to burn through PCr as well as O2, while an exertion against a heavy weight will probably not burn as much blood oxygen compared to PCr. You'd think speed work would blow through PCr faster than resistance work where the type 1 fibers play a larger role in force production.
Tough to say, if you're burning PCr at a high rate you're also trying to turn over ATP with every other system as well, actual levels of ATP vary only slightly from fresh to wasted.