Why does the meta key combo [ALT] + [0][0][9] change tab behavior?

This is on a Win7 x64 box running Office 2010 (specifically tab wasn’t behaving as it should in Word 2010.) The above “reenabled” the proper tab key behavior. x009 is {tab}, but not certain why inserting a tab would cause the tab key to start working… Can anyone clue me in?