The underside is dished to take a little bit of weight out of the base (also helps achieve even drying), and also a bit easier to keep the Base flat.
I went up to the timber yard last week and they had a massive Mulberry they had just felled so the guy gave me a couple of pieces of branchwood to play with. The timber is extremely gnarly and burry. I've never worked it before and decided to make a little Shrink Pot. It stands about 7" high. There are a few articles elsewhere on the internet, but basically- the log is hollowed out green, and a seasoned disc of wood is snugly fitted into a ploughed rebate near the base of the pot. As the pot then dries, the walls shrink radially and tighten around the disc, locking it in place with no other fixing.
The base locked in place by shrinkage.