The article here says that
This 20-micropascal reference was selected because it was the quietest sound pressure level that a group of normal hearing test subjects could detect.
which makes me think it should be the smallest sound that a normal human ear should be able to detect.
However, if we look at the db HL scale of an audiogram (0 dbHL = smallest audible sound at a given frequency for an ear with normal hearing), and a typical dBHL to dBSPL conversion table as given here, 0 dBHL at non of the frequencies are even close to 0 dBSPL. The smallest dBSPL value that is 0 dBHL is at 1000 Hz, and it is 7.5 dBSPL, which turns out to be 47.43uPa, much larger than the reference 20uPa. I expected 0 dBHL for at least some frequencies to be close to 0dBSPL but that's not the case.
What is the logic behind using 20uPa as the reference for the dBSPL? Is it actually the smallest sound an ear can hear, or is there something else?