I would guess that the signal strength changed for some reason. I've noticed that before: When moving from stone to stone, sometimes the signal strength changes and I have to wait a few moments for it to come back up to good accuracy again. It may be due to a satellite moving, a cloud overhead, or who knows what else. 20' is still pretty good accuracy from space to where you are standing.
You are correct in that GPS coordinates are not reflected in transcriptions. Transcriptions are just the words on the headstone, not its location. That is reflected on the cemetery map.