@Moiraine You're right - that could be any number of things, and that's assuming they actually stick to the full storyline and don't inject some element that wasn't from the books.
The low-hanging fruit option would be that it's the start of book two in the middle of season one. I believe "The Flame of Tar Valon" is the name of the first chapter in Book 2 when Siuan gets to Fal Dara. But, I agree with you. Book one is just a great story start to finish so consolidating that down among five episodes seems advantageous bordering on ridiculous.
That said, I also have a hard time imaging they'll do a book per season. I don't know what the rest of the series is like, naturally, but 13 seasons seems way too long. Consolidation will likely happen somewhere. There are quite a few elements of books two and three I think could be eliminated entirely though so I'm sure there are opportunities in the other books, as well.
Overall, I really like the series. I occasionally struggle with Jordan's prose - his world-building technique is enjoyable but it sometimes leaves me with a feeling emptiness, almost like less would be more. That doesn't happen often though. I mostly enjoy being totally engrossed in that world.