PGA281: Non-symmetric distortion performance

PGA281 or PGA280

The distortion performance I'm seeing using this part looks very non-symmetric and varies quite a bit part to part.  If I drive the inputs with a balanced signal I would expect the best performance, but instead the 2nd harmonic levels are as follows with a gain of 1 and a  5Vpp signal, no offset 10kHz sinewave input.

Balanced Input => 2nd harmonic at about -77dB, variation from 10 different parts is about -72dB to -85dB

Single-ended driving the positive input, negative input grounded => 2nd harmonic at about -72dB, variation from 10 different parts is about -70dB to -75dB

Single-ended driving the negative input, positive input grounded => 2nd harmonic at about -82dB, variation from 10 different parts is about -75dB to -90dB

So the performance seems to indicate that the part has the better performance if the input signals are mostly on the negative input.  Is this consistent with the parts design?

I couldn't find any distortion metrics on the part at all in the datasheet.  Is there any typical information available or an explanation for the widely varying performance from part to part?

Regards,

Jeff Lee