Average of AC signal measured with oscilloscope
Scopes have a time constant around 1 second, and you are looking at about 20 ms. Try looking at with .5 sec per division and see if there is not a ripple at lower frequency.  You can also check for amplifier offset by setting the trigger to AC and normal mode, and put the trigger level at zero.  If you don't trigger, the signal is never passing through zero so you have amplifier offset and scope error at this small signal.