Bolding text in TextEdit

This is odd behavior. I know that TextEdit won't let me bold text unless the font I'm using has a bold version (therefore, Geneva is out) so I set all of my text in my document to Helvetica.

Now when I select a line of text and hit command-B, all the text in my document becomes bolded! In other words, it affects all the text in my document, whether it's selected or not.

This goes against all my years of experience in using Mac apps. It happens wheteher I use the keyboard command or choose the bold variety in the Fonts panel.

I can't win for losing here. Is this normal behavior for TextEdit?? Is there some logic or rationale to the way this works?