I have a square taper sealed BB that feels buttery smooth out of the bike but it feels gritty when I turn the spindles by hand when installed in the shell? Why?

Possibly non-square BB shell, maybe over-torqued. Could be some dirt between the threaded shells and the bearings.
Actually in practice you will never notice the grit while riding. Probably chews up a few watts-machs nicht.

Ok thanks. On the upside I got rid of the creak by greasing the threads.