think about it
180 + 180 = 360 total degrees of rotation
90 + 270 = 360 total degrees of rotation
honestly i think a 90 revert 270 is much more impressive because it is so hard to spin back after the initial spin set, while i have seen shifties that look like 180 to revert 180.
and if we're bring physics into this, yes it would be impossible with a rigid mass, however because a person is not a rigid object, for the most part the center of mass will maintain the same rotation however you can bend/twist to complete a shifty.
either way, pep is superior to jesus, because even jesus tried to do a switch 90 revert 270 and couldnt.