My guess would be moisture. Call was somewhat wet when you started, then dried and warpped, after you machined out the interior. Less material will dry faster. Too fast causes warpage. Do you have a moisture meter? One way to know for sure is, take a blank, unturned, and put it in a toaster oven over night or longer. At low temp, preferably around 100 degrees. Then make a pot. See if it happens. I know some woods, walnut in particular, even tho dry, reacts to surrounding temp and moisture levels where I live, and can move enough through my finish work, that the surface will not fit right when all the finish is on and dried. I will have to rechuck, and true up the seat. Hope this helps.