I shot two boxes of 7.5 Swiss reloads today with the following results;
1. 43.0gr. surplus 4895, 155 gr. Hornady, ten shot, 3" group sitting 6" high and 2" to the left of the bull using a 6:00 O'clock hold.
2. 50gr. XMR4350, same bullet, ten shot, 3" group sitting 15" high and 4" to the right of the bull using the same hold.

My question is why the large variance, both horizontally and vertically, between the two loads?

Barrel harmonics.

Any good sources for further reading on "barrel harmonics?"

Don't know if there is a really definitive works about barrel harmonics other than to say it exist and its why reloading is a crap shoot from one rifle to another. You will sometime see considerable movement in your groups merely by changing the powder charge but not the powder. Fascinating, huh.