Event Viewer : CategoryMessageFile & EventMessageFile Difference

Hi,

I have an issue with the the working behaviour of CategoryMessagefile and EventMessageFile registry keys in HKLM\System\CurrentControlSet\services\eventlog\application\[source name]\

In windows XP, changing the value of EventMessageFileregistrykey, changes only eventmessage(description) field and likewise, changing the value ofCategoryMessageFile registrykey, changes only Task Category Field in event viewer

whereas, in Windows 7 64 Bit OS , changing the value of EventMessageFile registrykey, is affectingTask category field,  why so ??