You answer your own question: because there is more data.
For example, let’s say that we have a player ranked at a uniform flat 15k across all sizes. However, they have since improved their game and they win their next three games against three 13k: one on 9x9, one on 13x13 and one on 19x19. Naturally, their rank will go up, maybe to 14k on every size. But only the overall rank takes all data into account, which is why it adapts the fastest and is the most accurate. In this case, it will be the closest to or above the 13k opponents that were beaten.
If you have the qualifications, I’m sure the OGS team welcomes your contributions. You might start by familiarizing yourself with the work that has already been done and written about on this forum. e.g. these kind of threads as a starting point.