PJ2: That's the whole point: If the entered TOW is incorrect then the FMC would be expecting a different acceleration from the ACTUAL acceleration.
ie. For the given parameters (temp, pressure, N1 etc.) from which the thrust may be calculated, the predicted acceleration could be calculated for the entered mass. If the actual acceleration is significantly different then this would be an indication that the input mass is incorrect.
Of course additional factor affect the acceleration: Wind, runway contamination, variations between actual weights and std weights etc. But as a gross error check (in this case a 100 ton discrepancy) the difference between actual acceleration and predicted acceleration based on the weight entered into the FMS would have been significant.