The figures usually given for WLAN speed reflect the maximum _signaling_rate_, that is, the highest rate at which ones and zeros can be sent out during a transmission. This is also called the bit rate, data rate, link rate, link speed, or transmission rate.
Because of the difficulty of maintaining multiple two-way radio links, however, much of the data sent is not user data but "overhead" -- preambles, headers, checksums, requests, responses, acknowledgements, etc., that pass between devices but are never seen by users.
WiFi Information
The standards require that many of these be sent out at 1 or 2 Mbps for more reliable reception. There are also the inevitable garbled messages and retransmissions, as well as required short intervals between transmissions.
The result is that under average loads, roughly half of the "megabits per second" carry user data like web pages, email, files, etc. The speed at which user data is successfully transferred is known as _throughput_. Under the best of conditions, throughput on a WLAN is roughly half the signaling rate.