I understand what you're saying, and you're right, the Noctuas are specced at 22.6 dBA when they are at full speed (1500 rpm). For all intents and purposes, though, they are near silent.
When you measure the sound of quiet a room, you may get a reading of say, 27 dBA, but you might still hear the ticking of a clock in the background and this can be amplified in your brain quite easily, especially at night if you're trying to sleep. This is the domain of psychoacoustics, where the brain's interpretation of various sounds (particularly different frequencies) is completely different. You could measure two different sounds on a decibel meter and get the same reading, but this wouldn't necessarily translate well into how loud you would perceive the sounds.
I know this is a bit cheesy and cliched, but if you knew there was a clock ticking at 30 dBA, it would be one thing, but if you knew it was a bomb and you were in a locked room, it'd probably become so loud in your perception that it'd feel more like 90 dBA.