When you are in a multiplayer game, if one of the other players has the same ball type as you, the wear on their ball and the quantity they have will affect what the inventory displays about your ball. This has been a glitch for quite some time and should be ignored.
Check your ball quantity and wear when you are not in a game, or are playing solo for accurate display information.
This is what I saw in my inventory a few months back, I nearly fell off my chair! I would never have that many......ever!
⇩ EDIT : Thank you Shooter. : ) ⇩