Do you mean that they show in the 'manage devices' section of your account ? If so then that is different to authorising a computer. You can't see a list of authorised computers.
5 computers can be authorised : http://support.apple.com/kb/HT1420
10 computer/devices can be associated : http://support.apple.com/kb/HT4627
A computer must be authorised to be able to be associated, so a maximum of 5 of the 10 associations can be to computers. But a computer/device can only potentially be associated by using iTunes Match or automatic downloads on it, or re-downloading past purchases on it - Iif you just buy content on a device or sync it to it then it shouldn't become associated