I wouldn't say that's abnormal. I believe they even list a disclaimer about shrinkage, warping, etc... and I would expect a little movement in non-stabilized pieces like that. Keep in mind that a lot of this wood probably crossed over an ocean and came from a completely different climate, may have been cut wet, and may not have even been dried or acclimated by woodcraft. If using natural, non-stabilized materials, it is always best to get your pieces oversized so you can account for any necessary flattening/machining, and then give the wood a while to acclimate when you receive it (or have it stabilized).
I also agree with Ghostman and would make any future purchase from some of the fine sellers on this site. Not to say every piece of wood doesn't change shape in one form or another, but when you buy from guys who aren't processing multiple tons of the material, and individually handle each piece, you're likely to get a little nicer piece to begin with (not that your pieces are bad, but still...).
