Cannot use keystroke to assign ticket to user

When attempting to assign a ticket to another user, I cannot seem to jump to the intended user in drop down menu by typing in the first few letters of their name. Instead, if I type a few letters, the menu suddenly jumps to the top of the list and selects my own name.

This behaviour seems to be happening in both Safari 5 and Chrome 10, but not in Firefox 4 (all on OS X)