+1 to RE.
Also, Neck tension is critical if you're trying to get exact OAL measurements to the ogive. If you're not outside-turning the case necks you'll rarely get results any more precise that your current loads..... which is good enough for most anyway. (if one case mouth has more lube than another, it will affect OAL too. This goes back to about the same thing as neck tension.)
I doubt the Forster seater plug is contacting the AB bullet tip but it might be. The OACL will vary a bit because of any tip, even the AB tip but the ogive shouldn't vary that much for precision loads.
It's sure tough to troble-shoot when we can't get our hands on your stuff.
Maybe call Sinclair reloading and see what their techs say after you've tried a couple more things.
just my 2 cents,
Zeke