Well, that was the issue. If you cracked it out, of course they are going to re-grade it. Anytime you crack out a book, there are absolutely no promises that it will retain grade...just like when I crack slabs for signatures.
Had you left it in the cracked slab, I bet my next paycheck it would have retained its original grade. I've sent many slabs for reholdering...I never take them out of the slabs...and I've never had any problems.
Lesson for everyone? NEVER take it out of the slab when you send in for reholder service.