Herbert,
Let me reassure you that both of your 4070s behave as expected.
You can only apply the AC accuracy specification table if you are in 6 1/2 digits of resolution, your signal is greater than 1% of range and Autozero is enabled.
In your case, the range is 50V and a signal of 0.101V is below 1% of the range. The AC accuracy table is no longer applicable and the resolution will not be 100uV.
You should probably choose a smaller range such as 5V of even 500mV.
Autozero is necessary to achieve the highest resolution. In your experiment, when you disable it, you cannot accurately read your input signal: 0.0968V. When you enable it, you increase your resolution and your reading is correct. You should try lowering the range to 500mV and see if you can correc
tly read your input signal without AZ.
NIDMM 2.0 do not optimize AC measurements for the highest resolution like in DC measurements. If you require a resolution higher than the one given without autozero, you can enable autozero.
Good luck.