Very high (and very low) frequencies are hard to reproduce. My soundcard (or possibly the headphones) attenuates very quickly after 17 khz and becomes impossible to hear at 18 khz (or maybe it was 18-19).
Unless you know your system can properly reproduce those frequency ranges, alot of these tests could be inconclusive.
When doing these tests I would set the sampling frequency as high as possile, as 44.1k will start to alias at 22k and produce incorrect results (ie. you'd be able to hear something above 22k because of lower frequency aliases). Also be mindful that almost no one can hear above 22k, and definitely not above 25k, so this might suggest a problem with your setup if you can hear 22k+.
Most headphones are not designed to hit 22k+ either, and I'm not sure how this works, but I'd venture a guess that some piece of hardware will not allow frequencies that high to pass through or the cans will distort or fail to respond (or damage).