Your candid answer.  The only reason bevel base shoot more accurate than flat base is because the powder gas exts a dead sqare bullet base ---  IF THE MOLD IS CUT PRECISION AND IF THE BEVEL DOESN'T ROB A SHORT BEARING BULLET OF CRITICALLY IMPORTANT BEARING SURFACE.  I make them tiny because thats all that's needed to get results and it doesn't rob bearing surface. noticibly.
A little more technical explaination is perhaps in order.  -  With all base pour molds. bases do not solidify perfectly square.  I've measured about .002 to .003 on 44 caliber.  If the bullet has a slight bevel, which is on the SIDE of the bullet, the bevels will be perfectly square even though the flat bottom is out a bit.  When using RCBS or Lyman sizers the out of square bases, whether gas checked or plain base, can be made perfect if the stop plug is cupped out, leaving about a 1/32 flat around the primeter, which is printed into the bullet at least 80% of the way around the bullet heel or gc.    The Star does this fine with gc, pushing them nose first, but without checks, best precison is obtained pushing base first.