I no longer have a 30-30 of any variety so I am only speculating.
Are both bullet weights the same contour, round nose or flat nose?
I'm wondering if the seating stem in your dies fits the 170s more closely and is keeping that bullet straighter.

That's all I've got, good luck!

"Camping places fix themselves in your mind as if you had spent long periods of your life in them.
You will remember a curve of your wagon track in the grass of the plain like the features of a friend."
Isak Dinesen