No.. Its a theoretical weight based on the figures put in the FMS before engine start. Zero Fuel Weight, Fuel Uplift etc. Remember we use standard pax weights and actual baggage weights, but we dont weigh carry-on luggage or the actual potatable water/ catering as these are taken from the tabulated DOMs, so at best it's a fairly accurate estimate.
Based on computed take off weight less the weight of the fuel consumed, (which is a fairly accurate figure). I guess on the 767 ER with about 200 pax, assuming you are 10kgs out on each pax weight, the figure is accurate +/- 2000 to 3000 KGs but with 136T Landing weight, we don't care!