I do not really the answer.  ???
My guess is: it is due to the limit of the sound card (being used).
e.g. [url=http://www.ringbell.co.uk/software/audio.htm]Audio Frequency Generator FREEWARE[/url] can generate audio frequencies in the range 100 to 15000 Hz.
You can also check out [url=http://superuser.com/questions/238278/what-are-usual-maximum-output-frequencies-on-computer-sound-cards]What are usual maximum output frequencies on computer sound cards?[/url]