The problem is it gives 1 extra round... 1 Without the weapon perk, you have 55 round mag and 300 total rounds. Perk equipped, 100 rounds and 300 total. Extra heavy ammo perk equipped without the weapon perk equipped, you have a 55 round mag and 301 rounds total. The armor perk only gives 1 extra round no matter how you look at it. On rocket launchers, 1 round on a stack of 7 total of 8. On LMG, 1 round on a stack of 300 total of 301. On a side note, for other weapons (hand cannons, auto rifles), the armor increase to ammo stacks.

You get extra ammo in your inventory, not the clip. Maybe there's a limit to how much ammo you can actually carry and with all the upgrades you have stacked you're at that limit.