802.11 has gone through several revisions in its lifetime. The two most recent editions were published in 802.11-2007 and 802.11-2012. (Interestingly, both revisions have a connection to Aerohive. The task group that produced 802.11-2007 was led by Bob O’Hara, who now serves as an advisor to Aerohive; I followed in his footsteps and led the 802.11 revision task group that produced 802.11-2012.)
In 802.11-2007 and previous revisions, no changes were made to the clause numbering in the standard. New clauses added by amendments were frequently added to the end of the standard, but no attempt was made to keep the clause numbering consistent. One of the little inconsistencies that arose from adding on to the end of the standard is that 802.11-2007 clause 18 was added by 802.11b, but 802.11-2007 clause 19 was added by 802.11a. Even though the 802.11a project was started earlier, it finished later and therefore was assigned a higher number.
One of the many minor clarifications that were made in the development of 802.11-2012 - make the clause numbers follow a sensible progression. In 802.11-2007, the clauses jump around. They start off with general information, then describe what the MAC does, then how framing works, then back to how the MAC works, followed by how protocol layering works in 802.11, and then on to each of the PHYs in the order it was added. As a result, the two closely-related direct sequence PHYs are split by the infrared PHY, and the OFDM PHYs are split by one of the direct-sequence PHYs.
In 802.11-2012, the clauses are organized in a more linear fashion. The same introductory material starts us off, but protocol layering is moved forward in the document before the layers are described. The MAC is described completely by its own sequence (framing, how it works, how it is controlled, security, and mesh) before the PHYs are specified along their chronological progression (the obsolete frequency hopping and IR PHYs, the direct sequence PHYs, the OFDM PHYs, ending up with the high throughput PHY of 802.11n).
To illustrate the where the changes are, here’s a table.