My Surf PI Null Offset on Pin 6 of 5534 is different when measured with multimeter & oscilloscope

[Polymer]

I am getting my Surf PI ready for testing with oscilloscope in the sea to see what happens when dunked.

I had adjusted the null a while back with a multimeter when I had no oscilloscope.

Now looking at the trace of Pin 6 I saw that the null is below zero by a whopping 320mV....