What does error "POS HORIZ VARIANCE" mean?

When I do acceleration calibration I get this error at the end on my HUD. I am using mission planner 1.3.31. I have tried with all different hardware and still get the error. I cannot find any docs on it.

during a accel calibration it can be ignored, during flight it means that the gps velocity doesn’t match what the accels are telling it (this is bad)