I don't think I would make a change based on the flight of one arrow. Do you have another long arrow just to double check? I can't tell you how many times I've been taken on a goose chase by trusting one piece of data. Not just archery- cars, boats, any troubleshooting. I really don't know how much difference 20 gr would make.
I was trying to reduce the spine on some arrows and they were bending when I checked the spine. Pappy told me to use a scraper rather than sandpaper. The sanding was actually heating the shaft enough let it take a bit of bend with the 2 pounds of a spine checker. I guess I could have waited a minute to let them cool of but I'm an impatient SOB.