Don't read too much into it. It's a hokey-dokey tester. They work in new construction with all new products and open walls, because that work only tends to see a few types of problems. The product is tuned for that. When troubleshooting old work, they are wrong far more than they're right. That's bad because it send you on wild goose chases to hunt down a problem that isn't really there.
The original 3-light (red yellow yellow) testers were useful, simply because they were 3 neon lights each connected to 2 of the 3 pins. So just like that it would do 3 neon-light tests. They had a sticker that interpreted the 3 lights, and the sticker was pretty useless (all the problems I mentioned earlier).
This particular one eliminates the (rather useful) 3 lights and gives you a computer that gives you only the (rather misleading) interpretation. Send it back.
Honestly, the most likely problem with a "working, but tests badly" GFCI recep is a lack of a ground wire. On a proper 3-light "Magic 8-ball" tester that would test as "middle yellow only" (the hot-neutral neon).
It's perfectly legal to fit 3-prong receptacles that are ungrounded as long as
- a) the receptacle is GFCI-protected somewhere, AND
- b) the receptacle is labeled to indicate "GFCI Protected" and "No Equipment Ground".
Therefore it's quite possible that the real defect here is the lack of that sticker, the one that says "No Equipment Ground". That is mandatory. It doesn't have to be the hokey blue stickers you get in GFCI packages.