I also don't understand why it isn't always included as being part of the continuation novels. (It's like it's the forgotten Bond book.) But it is, after all, the authorized biography of 007, so since it is authorized, shouldn't it legitimately be part of the continuation novels?
Very tough to get a "definitive" figure when it comes to the question of "How many Bond novels are there?", even here on CBn (or, perhaps, especially here on CBn). Should Wood's "The Spy Who Loved Me" be counted? How about later novelizations (surely there's a case to be made that, say, "Die Another Day" is every bit as much of A Benson's Bond Adventure as "The Man With the Red Tattoo")? How about the new Moneypenny novels? And perhaps the likes of the unfinished-by-Fleming "The Man With the Golden Gun" should not be counted (I know, won't get much support for that one ).
'tis a tricky situation.
You won't get any support on The Man with the Golden Gun, but you may on Thunderball, which technically is a novelisation. It wasn't original and it's not really that much different than, say, Wood's two novelisations.
Personally, I don't count Pearson's or any of the novelisations as James Bond "novels" (Thunderball as the lone exception) in the traditional sense that Casino Royale is a James Bond novel. You got your traditional stuff Fleming, Amis, Gardner, and Benson, then your novelisations, and then the spinoffs, Mascott, Pearson, Westbrook, etc. I'm inclined to add Higson to the traditional - I don't know though, time will tell.