At risk of over-simplifying the issue, I'll submit that miticide X at dose
Y, failing to kill some number of breeding mites, creates an "opportunity"
for mites that are randomly able to survive that dose, and as each surviving
mite can quickly produce many progeny who also quickly mature to produce
progeny, the surviving line of mites tends to quickly dominate the local
population, as non-resistant mites are not around to breed or compete for
mates.  So, kill off 80% of the mites in a hive, and the survivors can breed
fast enough so that the mite population rebounds fairly quickly, as if
nothing had been done at all.

Bees, on the other hand, have a much smaller number of "breeding
individuals", each slowly producing a much smaller number of breeding
individuals, so a lucky queen, able to produce "resistant" progeny would be
far more rare than for mites.  So, kill off 80% of the workers in a hive,
and that hive will need extensive care and feeding to simply survive the
coming winter.  Even of that queen might be producing "resistant" offspring,
unless the majority of her offspring are resistant, the hive is still likely
to be doomed from the pesticide kill, and the "resistant" queen is lost
along with her hive for simply not having enough offspring that can survive
to maintain a critical (thermal) mass to cluster and overwinter.

So, beehives are simply more "fragile" in the face of pesticides than their
"robust" varroa infestation, and we have not even started on the issue of
maturation of a mite from one generation to another.

Yes, this is a massively oversimplified "model".  Most all models are.

An old joke about models:

Milk production at a dairy dropped, so the farmer wrote to the local
university, asking for help from academia. A multidisciplinary team of
professors was assembled, headed by a theoretical physicist, and two weeks
of intensive on-site investigation took place. The scholars then returned to
the university, notebooks crammed with data, where the task of writing the
report was left to the team leader. Shortly thereafter the physicist
returned to the farm, saying to the farmer, "I have the solution, but it
works only in the case of spherical cows in a vacuum".

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