There is so much more that goes into determining a handicap than just looking at a raw score over par. Lots of people don't understand the process and simply think if they shoot an average of an 82 on their par 72 course they are a 10 handicap.....but it's not.
My cousin thinks he's a 3 handicap because he average 39 on his local 9 hole course. The course has a very low slope rating. His "3" would probably translate into more like a 10 plus. He's still a decent player but no where near what he thinks he is.
Then remember that USGA takes your best 10 out of 20 scores after accounting for course difficulty......You can throw a few 90's into the mix and they simply disappear. Then that number is further multiplied by .96 to lower it even more than your best 10 adjusted averages. The reality is that you will play to your course handicap less than 25% of the time.
Compound the issue further when all of your posted scores are on your home course that you have learned how to play very well. Course knowledge is very valuable to help lower scores. There is almost no way your local course handicap will travel to new courses where you don't know the greens or trouble areas. The first time I played my parents course I shot in the 90's even though my course handicap was about an 11. My index was 8.5. That 140 plus slope was loaded with trouble everywhere that I wasn't familiar with. Add false front greens and crazy uphill/downhill/sidehill shots I didn't have a prayer. I can now play it in the 80s but still not to my course handicap.
So just judging a guy by his swing or performance on a single round is absolutely no measure of how he might play most of the time.