There is a lot in the book that doesn't reflect what was shown in the movie. I think the book was written and released a while before production of the movie began. Consider the novelization of ANH to be the final rough-draft.
There is also a lot in the book (at least the copy I have, which I bought in 1977) that contradicts other events that were revealed later.
When you read about all the drafts that Star Wars went through on it's way to production, simply because there were so many ideas that were impossable to pull off with the cinematic techniques available at the time, the loss of two squads of fighters is no surprise. The ground battle at the end of Episode 1 is a left-over from an idea G.L. wanted to use in the original movie, something about thousands of wookies overrunning an Imperial prison base (later to be the Death Star) and battling with an equal number of Imperial troops. He couldn't film it properly back then on the budget he had, so he dropped it and made it a space battle instead, which could be pulled off.
And the rest is history, luckily for all us X-Wing pilot wannabes.