Just an update here, after doing the area measurement in KLayout, I have gone back segment-by-segment, zoomed in on the intersections of waveguide polygons and quantified the sum of errors at the middle of each waveguide. My worst case asymmetric MZI length difference vs. target is still 5nm. So, I don’t think there’s an underlying accounting with a resolution much better than the GDS grid that causes the errors to cancel out once polygons are merged.
I would really value your advice in addressing this. Otherwise I will have to do very small adjustments to each AMZI and remeasure using this slow procedure to see if it’s possible to convergently reduce the error, or if it’s random each time.