It's because the mouse auto-allocates it's battery amount per user, so if your username uses more battery than your gf's user, yours will obviously be less than hers. Every wireless mouse does this. The cool thing is, if you have an identical username on another computer and attach the RF receiver there, it will remember the battery level found on the other computer and vice versa. This is because there is bi-directional data flow between the receiver and the mouse much like a printer but it's a bit more sophisticated because it understands Windows user data.