A correction to fleet weight usually may be because one airframe has some repair work or additional equipment installed or some structural items changed, be careful thinking this correction is not true for a reason.
I agree in regards to standard weights, actual bag weights would be a great starting point, as passenger weights cannot yet be given more accurately. However, an increase seems to be required as in general people are getting bigger.
It's been some time since I was sharp on, then EU-OPS. I tend to remember 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? Our airframes are quite old and all patched up in different places. It would seem unlikely that 9 out of 10 are on the kg same weight, while one is 660 kg heavier. It leads me to believe that it just didn't fall within the margins to use fleet weight on that one.
In Split the other day our dispatcher gave us the weight of what they had loaded. It didn't match with our loadsheet, that had the pieces at standard weight. Apparently they use actual weights at that station. We were 400 kg heavier than our loadsheet said.