This is because the buttons on the shirt actually do not have a specific seperate texture assigned to them.
You would literally have to do a brand new uv map on the model...re-rig it and then do a full custom texture that way to do the buttons properly.
The buttons basicly are semi-transparent and they simply use whatever texture the shirt does as it just shows behind the buttons giving the illusion that the buttons have their own texture.
It is just a trick some modelers use to save time when making a game model since most people don't get that hung up on little details like that....I do however but I am a different breed of modeler too
