Everytime you open the inventory screen the mouse cursor will move.
If you open the menu and your cursor was not perfectly centered it will move double the distance it is off center.
i.e If i leave my cursor 100 pixels off to the left when i close my inventory, when i open my inventory again it will 200 pixels off to the left. This will continue to double each time the inventory is closed and opened.
This issue also exists in ARMA 3, i hope it will be fixed with the new UI.