Hm, while I understand what you mean about where the actual energy goes, my understanding of how the costs are accounted are that there isn't any separate metering for "electricity out" vs "electricity in".
The Solar out actually runs your meter backwards, so does actually reduce the amount the meter shows you owing to charge you at the retail rate. There is no metering present to make it possible to charge you for the 100kwh you used at one rate, but then you pay you for the 100kwh you generated at a different rate. The electricity you generate runs the meter backwards, and reduces what you are charged for consumption -- which monetarily means you are "being paid" for it at the retail rate.
But if your meter goes negative in a given time period, they can tell at the meter you actually generated more than consumed, and could pay you out a different rate for that. (Or as OP notices, in some places, they allow you offset your use, but don't actually pay out for net negative at all. )
This is why the metering is called "net", it's your "net" use, consumption minus generation. I believe that's how it works for my friends here in Baltimore who have solar panels.
Are there other places in the USA where they install more sophisticated metering capable of separately metering out and in, and charge different rates for it?
Or, anyway, since we're talking about California, if anyone wants to clarify how it actually is going to work in CA, rather than speaking in hypotheticals based on our theoretical understanding of how things work, that would be even better!