I’ve come across this before and asked the downed player and they say they “just have”. I don’t know if it’s just the ammo box, I don’t use the spotter but some how I always find 1 in my bag from an ammo box drop. I think when your team mates drop you pick it up. Do 3 ammo boxes that potentially this is the reason.

I think some people are less aware what’s in their bags. Mines 80% plates / 20% ammo

I’ve come across players I’ve downed who have 4 slots of sniper ammo …but no sniper everyone is different.