The standard passenger weights used are average weights. There will always be some who weigh more and some who weigh less. That spannersatcx weighs more than the average weight used does not mean that the 399 remaining passengers does too. In fact, the more passengers the less deviation there will be between the actual and standard average.
Unless they all carry X kg of undeclared stuff in they pockets in which case the actual average will be X kg higher.