Different ears, different perceptions. I don't know. When I test frequency response I run a pink noise track and run a software EQ that is for all intensive purposes infinite in adjustment. With most earphones I will EQ something. With the CK90Pro, I have no place to add or subtract to fix anything. I could say I might add a couple dB at 30Hz, but at that point it gets hard to discern. The RE252 is very linear, but I do EQ sloped slightly down, i.e. is slightly bright stock. The result is pretty much a flat line across the spectrum with only a little extra adjustment at the ends. There should be some care in sine sweeps. Higher frequencies are automatically louder because there is more there. You can EQ with frequency tests, but I find it less consistent. I prefer pink noise. For one, you have all frequencies at once so you can immediately hear whatever you want in real time. A pink noise track is also equal in intensity across the spectrum so a 10kHz tone is the same intensity as a 100Hz tone because it runs a 3dB/octave cut across the board which counters the area gain under the curve as the octaves go up. Basically 100Hz-200Hz has twice as much there as 50Hz-100Hz, and for a normal sine wave, this holds true because the area under the curve is relative to octave. I toyed with test tones some time back, but once I stepped to pink noise I never went back or to anything else. Starting out it takes a while to get accustomed to hearing a mass of sound and discerning a number of frequencies at one time. Starting out it could take an hour just to be able to EQ something reasonably well once. After you do it a hundred times you can do it in a couple minutes. This is something I've used religiously for my car audio setups. I've run a couple dozen drivers through my car and had to match different tweeters, mids, subs together, evaluate driver performance, pick workable x-overs, balance out everything, EQ each side, and time align everything. Pink noise made the job easy, and I've stuck with it. Once accustomed to it and some experience under your belt, it's a very quick and repeatable tool. The only downside is if you don't use it for a while, you're a bit sluggish wrapping your mind around it again. Just starting out it's much more so, but I value it much more than test tones or sweeps.
I do agree, different ears, different response. Even one's own ears change over time, and our minds have little sound balance and tends to auto tune to what we're familiar with. I kind of see pink noise as a cleansing device to help neutral out our perception because it has no bias. Even volume level changes reshape the perceived loudness across the spectrum. I tend to EQ at my normal listening levels to ensure what I tune to is what I would normally be at when listening to music.