Exactly that. The OPT calculated VREF30 is based on your planned TOW as entered, the FMS displayed VREF30 is based on the current a/c weight which at this stage will be greater than planned TOW as it includes the taxi fuel allowance. Under worse case scenario the VREF will increase by 1kt for a weight change of 3.3T, therefore, the most it could be with a sensible taxi fuel allowance would be 1 kt higher and 0 kts less. Hope that makes sense?