I'd suspect one of two problems....
Bullet lube build up in the seating stem area...I clean this out periodically with WD40 or brake cleaner.
Your cast bullet's ogive is varying. Cast bullets vary in length more than jacketed and this could be your problem. Tho it shouldn't change actual seating depth much if at all.
I find that jacketed bullets vary far less than the cast variety, but in reality, in my experience, it doesn't seem to matter much in my groups shot from rest. But you're experiencing upwards of .030"+ seems a bit much....
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