I had similar issues with SFO-LHR on August 14th, and the issue (probably) was that the result had mixed cabins (business SFO-LAX, economy LAX-LHR), and the airline site (Qantas) didn't obviously display the same result. Probably it would have if I'd finessed the search on Qantas bit more, but I don't think your target novice user would figure it out.

As a side note, I despise seeing mixed cabin results for routes like that when I'm searching for business/first, where it's one hour in business and then 10 hours in economy. As far as I'm concerned that's an economy flight, or "economy mixed" at best. I know this is a standard practice for travel sites, this is just my particular hobbyhorse to rant about when searching for points flights. I'm guessing the "premium" slider somehow is supposed to deal with that since bumping it up removes that route from the search results, but the intended use of that slider is pretty nonobvious to me.