Why would you think they would only exaggerate by 4mbps. The 54mbps is a massive lie. They do things like add transmit and receive together. This would be like calling a gbit Ethernet cable 2gbit.....except Ethernet can really transmit 1gbit and receive 1gbit at the same time. Wireless can not so you speed is a least cut in half. Then is assume no interference at all. I forget what 54mbit connections tested out at. Your 17-20mbps is pretty good I suspect.
The lies have gotten even more massive as they inventented the newer cards. The router that have numbers like 3100 etc are massive deceptions. These are lucky to run connections to end devices at 250mbps. You are kinda lucky you get about 1/3 the rate they claim the newer stuff is lucky to get 10%.....the new routers have multiple radios and a single pc can not use more than 1.