Cookie, you are correct. The issue on this one is probably that it was clamped one side only, but also could have been that the thicknesser bed was not quite parallel so the timber is clamped perfectly together and has now formed a radius as the timber was not uniformly thick.
It's no biggie to fix, Lance. You might need to fix it first before you go on to the sub frame, so you've got a level surface for the mortices, etc...
Episode 11. Flattening the workbench
You do want a flat top (out of wind and no cupping/crowning) - makes it hard to get anything flat and straight if the bench isn't flat first, especially on thin stock that flexes under pressure.
Looks good - less than a term to almost finished. You should get there easily.
Keep up the good work.
Cheers,
eddie