Macready uses ammo supply differently when equipped in power armor

I just told Macready to equip some power armor, and now he appears to use his ammo supply differently than when out of the armor. Specifically, when out of the armor, I can equip him with a weapon and give him one unit of ammo for that weapon, and he'll have an infinite supply of ammo for that weapon. When in the power armor, however, he'll take one shot with the weapon and immediately switch back to his default sniper rifle. This occurs no matter what type of weapon I equip for him. Is this a glitch, or is there something I'm missing?

EDIT

I've confirmed that when he's in the power armor and I give Macready more ammo with the weapon, he keeps the weapon equipped until he uses all of the ammo. Again, glitch or by design? I should add that I'm level 89, if that matters.

EDIT 2

Ok, so I don't know what I was thinking. I verified that I was wrong about Macready having infinite ammo when out of the power armor. When traveling as my companion, his ammo is used up for any weapon I equip for him. In any case, the answer provided gave a link which describes how companions differ from settlers, which was appreciated.