Sometimes enemy bleeds but no damage is done

When I play multi-player source game sometimes I have situation that I can see blood from hit but my opponent still has 100%HP. It is especially visible in CS:GO and DayOfDefeat.

Why this happens?

This could be caused by client vs server hitreg issues. In this video you can see that sometimes (especially when you play with 100+ ping) csgo server and client hitboxes will differ, this causes your client to think you hit the opponent and display blood, while the server doesn't think you hit the opponent and in turn you get blood but no damage (and also if you're close enough you wont hear any sounds of headshot or body shot or anything from hitting him)

You probably don't hit his hitbox. This can be caused by you (not hitting hitbox) or your internet (lag).

If bleeds when you hit him on your pc and not on server (lag) or when you shoot his model, but not his hitbox. Image courtesy Blankooie.