Another example of the discrepancy is, of course, Green Dot speed, which is defined on the A320 and A319 as [GW in tonnes x 2] + 85 (knots).
In my experience the PFD displayed Green Dot significantly higher than the MCDU, which simply calculates it on the above formula, using the crew-entered ZFW plus the fuel remaining. The PFD value, IIRC, uses a gross weight calculated by the FAC from parameters including EAS, AoA and normal acceleration.
Presumably the possible implications of a difference between the two figures is that either the loadsheet understates the ZFW, the FQI is under-reading (very unlikely), or the aircraft performance is degraded.
In our operation, any difference greater than 10 kt (implying a possible mass discrepancy of more than 5 tonnes) had to be reported.
PS
The above was posted before reading CaptainMongo's posting. Airframe icing would obviously be one possible cause of performance degradation.