You are cherry-picking flights. There's only U bucket available on the 8:37AM DTW-SFO 4/27 flight. There's V bucket available on 4:00 PM and 8:34PM flights and you will end up with exact same $454 fare roundtrip if you return on one of those. 8:37AM flight on 4/28 also has V bucket. Not surprisingly, the early morning Monday flight from DTW-SFO is going to be quite popular with business flyers and DL is limiting inventory in lower fare buckets on that flight (even well in advance). Out of the three bay area airports, the cheapest is almost certainly going to be OAK. Unlike DTW-SFO/SJC, all fares on DTW-OAK are one-way fares (due to heavy WN presense) and start at $277 roundtrip. Just had a conference in San Francisco with Sunday departure and Wednesday return and OAK was easily the cheapest due to cheaper fares on DTW-SFO/SJC having a roundtrip purchase with Saturday night stay requirement. Yield-wise, SJC might be somewhat behind SFO, but if it were drastically different, I doubt DL would keep the non-stop flight on the route.