Curl,
Yes, I've had a couple of doubles do strange things not exactly the same as you describe, but along the same lines.

With your 300 it sounds to me like the lighter bullet- faster velocity loads are effecting the barrel movement to a greater degree than the slower 200 grain load.
Call it "regulation energy" for want of a better term where there is more of it (in your 300) with the lighter bullet weights than the heavier ones.
The old "barrel time" system of working up loads often goes right out the window with the smaller doubles IMO.
With faster cartridges in doubles, I think bullet acceleration speed down the bore can have more influence on where the bullet impact ends up than the barrel time/bullet weight system which usually plays a major part in many large cal moderate velocity doubles.