The gear teeth are helical so when you pull out the cam it will twist.

The cam will twist when you attempt to re-install it, so you must lead the desired position upon installation.

When the engine is at actual top dead center on the compression stroke, the cam mark will be down.

But you can re-install the mark in alignment with the cam follower hole which is more accurate. The cam rotates half the speed of the crankshaft. So if you then turn the crankshaft one full turn, the cam mark will be down with the crankshaft at top dead center.