It's just that they also use letters for clarifications, incremental updates, and in a couple cases tangential committee work, so 802.11 skips so many letters not because they weren't documents but because they were documents that didn't spawn a "new standard" outright. Some of the "in between" letters are used for interesting things, and/or are useful stepping stones in seeing the evolution of the standards that made it to "consumer adoption".

(Hence why it was maybe smart of the Wi-Fi Alliance to disambiguate IEEE procedure from marketing names for consumers, but also why they don't seem to be having much more luck with it than say the marketing names of cellular networks, because standards don't evolve "linearly".)