Yes, it is a curious phenomenon of audio perception. We've all had this experience: a part isn't standing out enough, so we give it a little boost and then we're happy. Sometimes the boost only needs to be 1db to make us happy. But ask yourself if a casual listener would ever notice a 1db difference. The answer is no. So why did it satisfy our own ears? Because we were
actively listening for the difference, and focused on the part. To achieve the same effect for the casual listener, you have to dramatically force their focus onto it.
Another example. I had a minor tambourine part in a song. A friend listened to the track and commented the tambourine was too loud. As soon as he said that, I was aware that it was indeed to loud and too piercing. From then on I could not listen to the song without being annoyed by that damn tambourine. But previously, I'd listened to the song a hundred times and had actually
forgotten that it had a tambourine in it at all!
It's just how our auditory sense works. It's how we can hold a conversation in a noisy room. Or how, when hunting for supper, our distant ancestors could pick out the footsteps of their prey among the other sounds of the forest.
It's also the same mechanism that keeps us from going insane from the constant barrage of extraneous sounds in our environment. I live a mile from the freeway, but I usually don't hear it. At least, not until I think about it - then suddenly it's very loud and very annoying. So I try not to think about it. Oh, dang it, too late - now I'll be hearing it all day!