When I am in the Equipment screen and I am giving my units what I want them to carry and then decide that I would like to put the ammo in a different container so that I could put my smoke grenade and I. Grenade in the belt container. I click on the ammo to move it and the tab changes to the Primary Tab from the Misc. Tab, usually. But the thing is, is that I am on the Misc. Tab for a reason and dragging the ammo out of that container so that I can buy something from the Misc. Tab.
The only time it makes sense to change the tab is if I click on a weapon, which I most likely will just click that Tab to get that ammo type if I decide I want more ammo for that weapon later.