That's comparing apples to oranges.
Before deciding what to expect you should go to the effort of learning exactly what is running at 54 Mbps, and how that affects your use. When you make "measurements", you need to determine what you are measuring, so that you don't measure apples and compare to oranges...
All of this information is available on the web... but you will have to make an effort to find it. Use google, and read. About all I can do is point you at some basics...
What is a "strong 54mbs connection"?
So, you measured through put!
You accept that on a 100 Mbps ethernet! Why??? ;-)
Do you mean your throughput was 11 Mbps? (How many other connections were currently transferring data at the time you measured this? And just how did you determine it was 11 Mbps?)
Nope, it don't.
Notice the nice round comparison? Twice as many connections transferring data, exactly half the throughput? Works pretty close to that every time.
Wireless 802.11g provides 54 Mbps total but spreads it over however many connections there are. It is also simplex, so it goes at 54 Mbps in one direction first, and then at 54 Mbps in the other direction. The overhead is large, and the result is that throughput just about half the single direction data rate. You won't see better than about 23-26 Mbps over a 54Mbps wireless link. If you have two connections, that will be divided between them, and you will see about 12 Mbps.
I can't tell what is causing your initial drop to 10-11 Mbps, but it could be a number of things. It can't be that it is operating as 802.11b, because you necessarily have at least a
24Mbps connection.