Dear Nathan,
Your observation is correct. When you enable Net Metering on the Utility Rate page, SAM does the accounting calculations on a monthly basis, and shows the monthly sales/purchases with and without system values in the hourly results as monthly totals in the last hour of each month.
The "Year 1 electricity to/from grid (kWh)" output variable shows the hourly flow of kWh to and from the grid. Because the value of electricity depends on the relationship between the total monthly load and the total monthly system output, the model does not know what the value of electricity is as it steps through the hourly simulations.
Best regards,
Paul.