The bars don't indicate a single factor, but all network quality factors bundled together. That includes ping/latency, but also jitter (ping stability), upload and download bandwidth and bandwidth utilisation, signal-to-noise ratio, and other things - I'm not actually entirely sure what the five parameters the GT network test shows you are, but I'd wager that four of them are in that list.
You won't show a perfect connection to yourself all the time, because you might have a high bandwidth utilisation (someone else in the house watching Netflix), a spike in jitter (someone else in the building using a vacuum cleaner or microwave if you're on wireless, or just a noisy line), or any one of a whole smear of factors. You also might show to yourself as two, one, or even no bars if you're the only person within 4,500 miles of you in the lobby, even if you have a stable, wired connection with a ton of bandwidth and nobody's on Disney+ through your router.
There's no reason why a player showing to you as one bar should also appear to themselves as one bar, although there's a good chance that they'll not see five bars, and probably not four bars.
