Can someone clarify this for me, I thought fare class availability was per flight and as long as you can get the same fare class on all legs you could purchase that fare class, but I just saw

9:30am Flight 1 (L9) - 5 hour layover connect to 4:30 Flight 2 (L9) - L fare class = $510

12:30 Flight 3 (L9) - 5 hour layover connect to 6:45 flight 4 (L9) - L fare class = $510

But Flight 1 - 2 hour layover connect to flight 4, shows both legs as (L0) and prices as a S fare at $690?

If L fares are available on both flights shouldn't I be able to buy the connection I want on an L fare?