Hi Greg,
If you're 100% sure nothing is clipping (either prior to or) the A/D, then the only logical explanation is the audio interface is defective. If you zoom in on the waveform, are the peaks "shaved off" (appear flat)?
If so, that's a clipped waveform. The front end or A/D would be the culprit.
If you have a sine wave generator, you could record that... and see very clearly if the peaks are shaved off (clipped).