No two hives are the same and the interior organization, demographics and culture vary as do the frame condition, contents and spacing,. The position and tightness of the cluster and amount and stages of brood and degree of activity  in it vary.  This affects distribution of the chemical and also of the target mites.  Locating the strips is a bit of a crapshoot, too since clusters move.

When first offered, in Canada at least, the efficacy of Apivar appeared to be quite even, possibly due to overkill in some hives and barely adequate kill in others, but as efficacy declined efficacy in some hives became obviously reduced.  

Medhat did some experiments a decade or so back and when I examined his data although the *average* looked fairly good I could see immediately over a number of hives Apivar control was starting to look like a picket fence with boards missing.  In some, Apivar kill was high, but in others  Apivar was on a (low) par with Apistan for kill. 

Over a few years there is a huge difference between 98% kill and 95% kill and over time we were seeing much lower numbers on average and increasing individual failures.

As for formic, Jerry has some really good demos of how quick and smart bees are at deflecting smoke.  I imagine formic fumes are managed similarly so the location and number of of the bees, stores, amount of stores, and of the treatment will affect outcomes. Weather is also  a big consideration and varies even within a yard.

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