Musical notes are rarely (never?) a single frequency waveform - there are layers of ordered harmonics. Some peoples versions of perfect pitch often seem to have

Musical notes are rarely (never?) a single frequency waveform - there are layers of ordered harmonics. Some peoples versions of perfect pitch often seem to have some reliance upon the particular harmonic structures that they've trained their ear to, e.g. a particular instrument. And it makes sense that those extra layers of info and interplay would be useful to the brain as it makes its analysis. As opposed to the brain entirely brute-force-counting a notes primary frequency in some manner. Interestingly, other aspects of music and listening can develop great levels of aptitude too - not just absolute pitch. Relative pitch is a common one, closely related to harmony. Rhythmic analysis is another - a suitably skilled listener or musician can audibly derive the exact rhythmic structure of extremely fast and/or complex pieces that would boggle the mind of a casual listener.