You get 25 supply left in the supply truck, the only option you got is "unload all (25)". It takes as long as too unload a full 1000 supply point truck.

The loading/unloading time should be based on the number you load, not the option you use ?
Basicly my guess is that everything that is not "prebuilt with a menu dedicated" (500 / 200 / 100) will not take into account the supply you're moving IN or OUT, which is wrong... even the "(un)load all" should take into account the numbers of supply still left in the truck.
Also the option dedicated to "unload 100 supplies" disappear when there is 100 supply only left in the truck.