Understanding fare class availability
 

I'm checking availability on the routing KNO-CGK-LHR on GA on 17th April.

The KNO-CGK leg shows L9 (L is the cheapest class) H9 S9, etc., but CGK-LHR shows L0 H0 with S9 the cheapest class.

However, if I check the flights separately, both KNO-CGK and CGK-LHR have L9.

Is there a particular reason why this would be?

not specific to GA but airlines have different availabilities for different routes, even if it is the same plane, it's a different market in terms of supply & demand and what people are willing to pay

I want lhr-cgk returning kno-cgk-lhr, which is £120 more than lhr-cgk-lhr, meanwhile the price of the kno-cgk ticket one-way is only £50. It's UAM so makes it more complicated otherwise I'd just book two tickets.

Do not book the legs as separate tickets. If something goes wrong with the first flight and you do not show up for the second flight then you are treated as a no-show for the second flight and will forfeit the fare.

As noted above, this is driven by complicated and dynamic demand/supply algorithms and tweaked by revenue management. The armchair EF user is not going to crack the system of why a certain market sells a different combination of fare classes for different prices and different rules from another. If you want to save £70 by self-connecting without any onward protection should there be a problem, then that is your choice. You may find it comes at a great cost.