Those are the "signaling rates" (11 Meg and 54 Meg).

To make an otherwise unreliable medium (RF / Air) more reliable, the data is combined and duplicated with other data / signals that effectively reduce the actual data throughput by roughly half (actually a little less, then there's the other protocol overhead).

Also, in an 802.11g environment, if the AP sees any (even a single one) 802.11b client, by default, it will slow the throughput for every client even more because it must now watch both both schemes (g and b) .... clients on one can't see the clients on the other, so the AP initiates additional signal coordination, which slows things down some.

The short answer is that it's done for reliability. If you tried to run wide open @ 54 Mbps, it's likely that things would run even slower (than 22Mbps) because of re-transmission.

Good Luck

Scott