The OGS used the EGF system in the past. Then at some point OGS changed to Glicko-2. That has an uncertainty parameter in each player’s rating (perhaps somewhat similar to the AGA rating system).
The EGF changed their prediction curve in early 2021 (from the green curve to the blue curve in one of my posts above), based on analysis of their historical data. That was part of a larger update of the rating system. I was part of the commission that made the recommendations for the update.
OGS also changed their prediction curve in early 2021, based on analysis of their historical data.
There was some contact between me and @anoek at the time, but we both did our own analysis on our own data sets. So I suppose the reason that both predictions curves are similar is due to our statistics being similar. At the time OGS also received the historical data of the EGF to analyze, but I don’t think it affected the OGS rating system update.
The resulting prediction curves of OGS and EGF are not completely the same BTW. They diverge significantly for higher dan ranks:
