Just found this explanation in another thread:

"the issue being when I have set the vario to Netto in the
past, with the glider on the ground, the needle points
to about 1.5kts up....which I have just realised it
what it should do...as it is assuming if the glider
is maintaining altitude it must be in rising air eauivalent
to the sink rate"


It's making sense now. I catch on "eventually".... Ha ha!