(Why did my "TM" turn into "â„¢" in your quote?)
Because in System Preferences > Keyboard > Text I have the characters TM setup to substitute â„¢. That seemed a good idea at one time, but it has become annoying when using TM to refer to Time Machine. I will change that right now.
I've got the same pref set in System Preferences > Language & Text > Text, but it has never, ever worked for me, and I've long since forgotten that it's even an option.