Was on United Express 5379 tonight, SFO to ACV (Eureka-Arcata, CA). (These flights use the 30 seat turboprop EMB 120). I was on standby for the earlier flight, and was #3 on the list. When I got to the remote terminal, I asked the agent what the chances were. He said, "Since you're #3, if they take a full passenger load, you're on." Didn't know what to make of that. While waiting to see if I would indeed make it on to the earlier flight, I sat near the podium and there were several phone calls between the agent and ops (?), which kept changing the numbers: "We can take 27." "We can take 24." "We can take 26." Was a puzzlement to me. The earlier flight left w/o me when they settled on 27. When I boarded my confirmed flight around 8pm, I was told to take both carry-ons on the plane and not to check one planeside. This was unusual. Then we were delayed a bit while some other folks were asked to remove and plane-side check their carry-on's, and then, oddly enough, some bags that had been plane-side checked were brought into the cabin to be put in the little overhead bins. Finally all this musical baggage was over, and the door closed, and we had 27 pax total. Then captain came on and clarified all this. He said that since ACV is notorious for nightime fog (much worse than SFO), the pilots had to carry significant extra fuel for two approaches and then a return to SFO if they couldn't finish the approach due to low ceilings. Therefore, they had to check and re-check the ACV wx observations and forecast, and if too low (by whatever formula UAExpres measures that) then they would only take a certain percentage of the passengers and use the difference in weight to carry more fuel. He also said the reason for the luggage shuttle was that he had told the ramp agent only x number of bags could be checked plane-side. So they were trying to make sure that everybody got there with all their bags. (I assume that ops assigns an average weight per bag). This was my first time on this route (I fly alot on the EMB120 but on the more benign routes of LAX-ONT/SNA/BFL), but alot of the "veterans" around me were trading stories about how many times they had ended up back at SFO. (And, if your're wondering, the landing was uneventful, the 400 foot ceiling which was what was reported earlier had been replaced by patches of low level scud. And I have to say that I have flown at night to a whole lot of airports, both as an airline pax and a private pilot, and I have never seen runway lighting as bright as at ACV!).