I have made some tests and I suspect that the problem is that you are using RetrievaType=Compressed. I also received two values in this situation however after changing this parameter to InterpolatedValues, I have received the number of values defined on the NumValues.
Let me know if this helps you!
That is what I would expect as well.
When trying again and making sure interpolated was on it seems to work.
Probably my bad, but just seems to stupid.