A few interesting comments ..
May be a factor also if airline uses fleet weights which can define load sheet differences
Of no relevance to the topic. All fleet weight does is reduce costs a bit for the operator. The operational side of things should just be pulling the declared weight/IU for the hull for the loadsheet calculations - whether the data are fleet or individual makes no difference.
if 1 out of 10 of our 737's has a correction of +660 kg to DOW, but the others has 0 kg
I do hope this is a wind up. If the individual hull variations are significant, the benefit of fleet weights is lost in the operational cost penalties. Any sensible operator would schedule multiple fleets for the purpose of fleet weights in such a situation. No one in their right mind would tolerate that sort of variation across a fleet weight mix. More importantly, fleet weight protocols will involve a maximum variation of an individual aircraft from the mean. For instance, in Oz, this is 0.5% which wouldn't permit the scenario which you describe.
I agree in regards to standard weights
All standard weights should be based on periodically revisited statistical reviews. Provided this is done, and for larger aircraft, the use of standard weights usually figures reasonably well - swings and roundabouts.
I had a look at the loadsheet and saw that it was not the correct registation
That doesn't say much for the flightcrew's procedural discipline, does it ?
if at the actual weighing, the individual's weight does not vary too much, within margin, from fleet weight you can stick to fleet weight?
There probably exist various rules for fleet weight procedures. The idea of fleet weight is to use a suitable mean data set for a group of aircraft which fit within a prescribed scatter. If, on reweigh, an aircraft finds itself outside the boundaries then either the fleet weight data gets changed (after all, that's just a paperwork exercise for the guys in the back office) or that aircraft goes into another fleet or gets addressed individually.
It would seem unlikely that 9 out of 10 are on the kg same weight,
Ten nominally identical aircraft won't be sufficiently "identical" for all to have the identical weight and CG .. they might be reasonably close, but identical would be in the realm of fairytales.