You normally purchase ammo per ton (except for stuff like half ton MG ammo) and if a unit is out of ammo (and you do not have any stocks for it), you can go to the repair bay -> acquistion tab to acquire the ammo. However you will only order the specific number of missing shots.
E.G. in this savegame, the archer in the repair bay is missing 4 shots of LRM-20 ammo. Acquiring it from the acquistion tab will only order 4 shots, instead of the full ton, whereas if you order it from the parts market, you get the full ton normally.
I'm not sure if this is intended but this is just making it harder to acquire large amounts of ammo from the acquisition tab.
save file
good for gaining XP for whoever is doing the orders though. It isn't just ammo though if memory serves.. I think armor is the same way though I haven't run across this in awhile.
This is working as designed. Acquisitions tab is meant to acquire ONLY those parts which you need at that time.