I was doing some searches and found that for the same flight, I sometimes get two different listings of fare availability. For example, a NRT-SFO flight on one itinerary will try to sell me W class but show R3, where on a different itinerary, it will sell it to me as S class but show R6. And the only difference in itinerary is the time of the next leg. How do you know what the real R availability is?