My understanding with the digital I/O is this. Pro gear uses 48 kHz, whereas consumer gear uses 44.1 kHz. Having read reports that the Ultracurve digital I/O wasn't so hot, I'm guessing that internally the algorithms are all based on 48 kHz, and if you connect a 44.1 kHz source, then it has to convert between the two rates. Because those numbers are not a simple ratio the conversion is not straightforward, it is easy to introduce artifacts in the process. It might be that an analog to digital conversion is preferable to a poorly-done digital-to-digital conversion.
I'm just guessing, but I believe this is certainly possible.
JohnR