AKAAB
Don't remember a limit set at Vmo -6 knts. As has been pointed out autothrust should limit the speed to Vmo. FCOM Bulletin 45 points out the tolerances for PFD protection speed indications. Although it doesn't quote a tolerence for Vmo from memory it too is +/- 5 knts. From what you say you have tried selecting speed above the limit to see what happens. Could it be that the tolerance was at the maximum? Don't forget that the FAC generates the speeds based on AoA and that a difference of just 0.3 degrees can generate a 3 tonne error in calculations.
