Interesting.
I wonder how accurate this is due to defects in the coding process and variability in the performance of sound cards/integrated sound.
I can hear up to 16,000 Hz on that page - but if I generate my own samples in Audition I can hear up to 20,000 Hz. Interestingly, I can hear higher than 20 kHz more easily, but they don't sound as high pitched - what I'm probably hearing is distortion products.
A long while ago I had access to a research grade sound system including a 500 kHz DAC, true analog low-pass anti-aliasing filter at 40 kHz, high-end 'audio grade' amp (Arcam, I think) and state-of-the-art headphones. I could hear up to about 22 kHz - but what was interesting was that something in the system was causing weird distortions at very high frequencies (I could see stray peaks on the spectrum that came out of the amp).