2/10 was definitely more than 20 at DCA and indeed they had an initial observation closer to what Dulles got. They then revised that observation but also acknowledged that they lost the snow board during the storm. Essentially they guessed.
This map doesn’t really tell the story that dc is on the wrong side of the thermal boundary on the hrrr for most of the event and gets saved by a nice backend band as the coastal pulls away. That’s too hard to predict.