The RMSE could be smaller if the drone generates better GPS coordinates for an axis that is not changing (which it could judge by looking at its IMU data). If the drone was really smart and compared the compass direction (E-W) with what the IMU is saying for the perpendicular direction (N-S), then it could average the GPS y-reading as long as the IMU keeps saying there is no N-S change. Then after a turn (two 90-degrees), it starts over and finds once again its going E-W, IMU says there is no N-S change so it averages the GPS y-reading again. But maybe the drone is not this smart. I would have to fly a mission with 1200' N-S legs to confirm this. Certainly the RMSE report seems to indicate this is what's happening. All my missions have shown over 2 times X error compared to Y error. If this is actually what is happening, then all missions flown parallel to longitude or latitude lines for the long legs will have lower RMSE. And missions flown at 45 degrees relative to longitude or latitude will have the worst RMSE. Is this what your missions show? Of course one needs to be careful to sort out other factors that can have a large influence (wind, lighting conditions, altitude, flight speed) before drawing conclusions. I try to carefully control these variables for my missions plus use better and better flight plans and have been rewarded with smaller and smaller RMSE. But currently I cannot prove that flying long E-W legs reduces the RMSE. But it not, then something else strange is going on.
This is what I was getting at.