GPO With Secure Filtering, Why do I need to add the computer?

Hi!

First I will explain my enviroment.

I have one OU, OU A with linked GPO, GPO B:

GPO B Status: Computer configuration settings disabled.

OU A contains Users

In the Secure Filtering, I remove Authenticate Users and I added a Global Group with some users from OU A.

GPRESULT shows:

GPO A     Filtering:  Not Applied (Unknown Reason)

But if I add the computer of those users to the secure filtering,the GPO is applied.

Can anybody explain me, Why!? Why do I need to add computer accounts to the secure filtering? The gpo computer section is disable, the OU contains users...
I can't understand.

Can anybody explain me this behavior?

Thanks!!