When you use unicode symbols (like , , , etc) in your username, instead of the symbols being a square (because i’d imagine unicode isn’t supported for the fonts) it turns your username into a random 3 digit number.
What’s weird is that whenever you join a different server, the number changes.
Source has it because it’s an engine from 2004, when not that many fonts supported Unicode. Steam is able to serve Unicode perfectly fine, it’s company the the application to handle it correctly.