to answer your question with some civility, the pistons will only come out the top as the main brg castings would foul the piston if you tried to remove it from the bottom!
It doesn't have anything to do with bores being 'fluted'(not actually sure what you mean there). The bore are in fact tapered, however the taper runs from bottom to top i.e larger at the bottom than they are at the top.....so they would actually go in easier from the bottom (if the main bearing castings were not in the way)

HTH
Pugo