I don't have the full answer here, but BA2780 is a near daily flight LGW to PFO, while LHR has a BA898 service on 2 days a week, Wednesday and Saturday. Today there is, therefore, ,a LHR service, but as it happens no LGW service. Next Wednesday it's the same again - LHR operates but not LGW. Though this thread focuses a lot on cancellations announced with 3 or fewer days notice - these show up in Flight Status - but there are quite a few cancellations made with 2 or more weeks' notice. These won't then show up on Flight Status and get removed from the Schedules web page in both scenarios. But when they were active flights the airports would receive a data feed with the original flight details, and then an update with the cancellation. Hence from PFO's perspective they see a cancellation and report it as such, but on BA.com it will be wiped away. The passengers presumably were all rebooked either to the LHR service or to the LGW services on the day before or after.