The circumstances in which the green/red bars show up is a bit weird to begin with and I don't think we ever completely pinned that down. It's not impossible that it measures them in some way relative to the system economy rather than the station economy, even if the underyling tonnage number is identical, for example.
When I was testing building Agricultural settlements, I did not see any significant effects on the Ind->Agri outputs of my Ind outpost which is isolated from weak links when measuring numbers, though I didn't think to check the green/red status.
Perhaps it is then just something that affects the green/red status and not the numbers as you say. If that's the case then it's probably done so people don't get OP BGS trading opportunities between two stations that sell to each other in the same system?
I just checked my Extraction T1 Surface Outpost, which has pretty colossal supply numbers compared to the Coriolis (Say, selling stuff with 5 million supply) and yet the same thing is observed here with those commoities consumed by Agri and Tourism being on red bars or no bars at all with lowered supply (Namely Palladium, Silver and Biowaste) while its demand for Agri commodities is also diminished. I do have to say that previously, when the whole system was exclusively extraction, literally every commodity on both pages was at a healthy green.
If it has to do with the supply of stuff from the Agri/Tourism station I imagine those numbers make a difference given the supply is higher per the station population, and settlemens always sell stuff on like 50 supply so they can't make a big difference on their own other than the weak links.
This is a picture of the demand in the planetary outpost, seems like the prices are also bad