They aren’t as accurate as when do properly by an audiologist, incorrect headphone volume levels and external noise affects the results.
I mean, it depends. Audiologists do testing with a specific goal -- to gauge how HL affects your ability to hear speech and other things in daily life. It's accurate for what they do, but it's really not that precise when we're talking about critical listening to music. And it's not like we're measuring electronics; there's inherently a brain and perception involved. I'd say adjust EQ to taste and not worry beyond that.
I have mild-moderate low-frequency hearing loss in one ear, and when I first noticed this and went to ENTs/audiologists, my experience was that it was fluctuating. But on audiograms, it really wasn't what you'd medically call "fluctuating," in that it was consistently 30-40db loss at the lowest frequencies, rising up to the normal range above 4-6khz. (My ultimate diagnosis is otosclerosis, a genetic hearing loss that is generally gradual and not "fluctuating.") I did some tests for myself at home, where I figured I didn't have anything calibrated to truly measure thresholds, only my "good ear" and my "bad ear," so I made up two tests:
1. The gain difference required to make a tone playing sequentially in my left and right ear match in perceived volume
2. The gain difference required to make a tone playing in both ears sound like it's in the center
I played tones on my computer through headphones. The results of these test did show that these things were fluctuating a bit at the higher frequencies, though they were consistent at the lower ones. Enough to be perceptible to ME, but not so much of a difference you'd pick it up reliably on an audiogram.
But these two tests gave difference results. Generally, the gain difference required to make things sound like they were in the center was lower than what was required to make them sound "the same volume" playing the tones sequentially. In other words, a tone could sound distinctly quieter when played in my right ear than when played in my left, but when I played both right and left channels, I perceived it as in the center! Weird.
I put in some time trying to figure out a precise EQ for my right ear, but ultimately I've gone with "simple is better," with just a low shelf boost and winging it on the gain. Generally it sounds center-ish, though it shifts a bit depending on the song and style of music and headphones, and probably my ear on a given day. Good thing nobody relies on me to have perfect stereo hearing!