I HAVE THE ANSWER!!
I have a linux based thin client connecting to a windows server via rdp. I have the extend problem in excel when using the numpad.
I too have struggled with this problem for many months and I fixed it by attacking the source of the problem.
The num pad minus and the minus on the main keyboard are different keycodes.
Remap the keycode in the registry and the problem goes away!!
Here is the regfile that will map the numpad - (minus) to the keyboard - (minus)
Windows Registry Editor Version 5.00
You will need to reboot but it works straight away!!