Just to explain it, due to HL's movement prediction code the movement you see is client-side, as it thinks you're able to jump. The server gets the information, deems it false because of the games coding, and you're put back on the ground in your screen. Since that info was not accepted by the server, however, you don't move in anyone elses screen.
How "high" you go is dependant on your ping- that is to say, you'll "jump" like normal until the server sends your client the information packet that you're not actually jumping, in which case it forces you back to the ground. Test it- in a server you're getting 13 ping, you'll only go up a few inches, but on a server in which your latency is 278, you're going to probably reach the apex of your jump before the server gets the information to your client that you never jumped, forcing you back down.
(I am aware that this is sort've old, but I feel as though an explination might satisfy future people who report this "bug.")