It is dependent on where you drop your supplies and where they will supply the most. They only have a limited range too. It is maybe possible you dropped the resupply only close enough to resupply half the strat's buildings, or maybe you were simply flying with .5-.7 country ENY.
Eny has no influence on resupply perks.
It's always 0.25x # of objects resupplied. Perk bonus for landing instead of ditching is 25%. Without the resupply cap, perk gain was only depending on the state of the factory/town you were resupplying. As stated earlier, the most objects I ever resupplied in a single drop was at a radar factory that was almost flat: 91. (22.75 perks)
Strictly in terms of perk efficiency, ditching at the factory was/is to be preferred over going back and landing.
On September 2, I made 7 resupply runs with a total time of 1 hour, 3 minutes and a gain of 103 perks.
The next day, 5 supply runs with 32 minutes total time and 43 perks
All with absolutely no risk. Compare that to ordinary bombing runs ...