Standard weights for pax are reviewed regularly and can be overridden by the loadsheet agent, a common example during training was if a group of Sumo wrestlers were travelling.. even here the crreection that you applied was an estimate. A colleague working for another airline which had a high traffic of Indonesian female workers (quite petite) applied for, and was granted a lower standard passenger weight for that sector. This resulted in more payload available for cargo.
When I worked for LH, the weights for short haul flights were higher than for long-haul, the rationale was that short-haul pax tend to have hand-carry luggage so this was factored in.