Hmmm, never thought to check mine either. I just checked them on my stone reference table, 9"x12"x2" thick granite. I use this surface for doing layup work on metal parts. The 6" and 12" sanding bars are dead nuts flat on mine. If it matters that much to you, at 1/32" out of tolerance, I would think a quick touch on a belt sander should true it up.
Edwin