Generic rifle bases vs higher end

[cfnewfie]

I've been experimenting with a theory lately about the accuracy and consistency of scope mounting. My recent experience was using factory bases on a 30-06 higher end rifle that no matter the adjustment and the fact the scope was remounted a few times using a leveling kit and 100m string test to confirm, that the bullet drop would wander to the left as distance increased. First thing was loading a string test for 4 distances, 5 round per range at a .5gr upcharge and they all wandered to the left. Wind was not a factor on that day. After switching to a 1 piece 20moa rail for that rifle my ladder groups now travel in a near straight vertical line. This leads me to believe that factory mounts were machined in properly or offset that caused my horizontal stringing on the ladder test climbs. Anyone else experienced similar accuracy performance switching to a higher quality rail?