FMC coding is done by your database provider based on information received from both your airline and official nav-data sources. I haven't seen that 265B coding yet, but we simply may have different database providers or aircraft performance figures provided by our airlines. Could you give a specific example?
We have two different landing weights, one is slightly over 66t and one is 65.3t. The first one can lead to the interesting situation that the landing weight is higher than the MTOW (which will change every flight according to actual weight).