If it's like my system, I think it's just the nature of the PW's inverter lagging the actual house demand changes. So if the house load suddenly increases, some grid energy is used as the Powerwall(s) ramp-up their output, and similarly if the house load suddenly decreases, some energy goes back out to the grid until the PWs ramp-down. So the grid basically acts as the shock-absorber for the quick house fluctuations, even if they're tiny. It likely all averages-out in the end, though on my system at least the "average" isn't truly 0 but around 20W drawn from the grid, so over the course of a day there's still ~0.2kWh seen by my meter overnight even though my PW banks all the energy my house needs for a night. But the Gateway UI sometimes shows some power going to or coming from the grid, and the REST API that I poll every ~2s constantly shows some positive or negative grid power value, I think the UI only lights up the grid path when this value is above a certain number (~0.1kW I think). In my case none of it is big enough to register visually on the Tesla app, though it does show the ~0.2kWh grid consumption every day.