It's a combination of booking class availability, fare rules and (I don't fully understand this bit) location.

So - although there may be available seats in a booking class that usually attracts a lower fare there may not be any fares with valid fare rules to allow you to book it.

So if a flight is H9 V9 N9 S9 Q9 O9 you may not be able to book the cheapest O seats because there may not be an Oxxxxxx fare that allows booking on that date, day of week, flight or combination of sectors you are trying to book.

Someone else may be able to secure an O seat because it may be the return segment of their itinerary (fare rules are usually based on the outgoing sector date), or they are originating somewhere else where an O fare is valid. (e.g. may not be valid LHR-LAX but may be valid ARN-LHR-LAX as the ex-ARN market may have different characteristics that mean you need to discount at different times of year)

To make things more complicated (and I don't fully understand this) - I believe there are cases where a flight shows O9 if you are booking in one market (e.g. USA) but is zeroed out if booking in another (e.g. UK).

That said....

I just looked at a date in July and one flight on a particular date was booking into V and the other two into L. Checking on KVS all booking classes look wide open and the fare rules don't seem to black out specific flight numbers (which is also quite common). There must be another factor involved...maybe it's the 'booking in a different market' thing.

All said and done - £650 is not that bad a fare for a non-stop to california in July/Aug.