Worth mentioning that the nominal speed of WiFi e.g. 11Mbps or 54 or now higher is exactly half the maximum achievable speed with a perfect installation using special test software.
802.11b - 11 = 5.5
802.11g - 54 = 27
I have no idea if this applies to later stuff, n, ac, ... but why would they change it if it has confused people so successfully for so long? Nor do I know enough about how 802.11 works to understand why this is.
I suspect that perhaps n and strongly suspect ac and greater are hardware dependent. Not all ac hardware will get 1200Mbps (or whatever it is).