diff alias

i'm using >bzr alias diff="diff --using c:\Beyond_Compare_3\bcompare.exe" to run external diff from tools like WingIDE and it works !
But from bazaar explorer the builtin diff will run no matter out. i need to run diff and than press external diff button and choose the external tool i wnat to use.
Why external tool run the right diff and the bazaar own tool ignore it?

NOTE : the external diff is configured and marked in user configuration