Do you have anything wired to the feed-thru lugs of your Span panel?
My guess is no.
It is my understanding that the feed-thru lugs power is not measured, it is computed, something like Power(feed-thru-lugs) = Power(input-lugs) - Sum( Power( circuit[1]), Power( circuit[2]), … , Power( circuit[32]))
Any noise or errors along that summation would cause a (presumably) small deviation from zero.
If I understand you correctly, you are seeing ~ +/- 10 watts which seems consistent with how I beleive the Span panel is reporting feed-thru power.