It's called the stacking effect. the key way on the crank may be of a bit. the key way in the bottom gear may be off a bit, the dowel hole in the top gear may be off a bit and the dowel placement in the cam may be off a bit as these things are less than perfect.
sometimes it stacks up to 10 or more degree's off.

I would leave your cam where it is you will like it with a tad more advance!