If your flight number changes in LAX then it is two segments, if you take the direct flight (same flight number) all the way to JFK then it is one segment, even though the flight makes a stop in LAX.

Edited to add: and in the first case (two flight numbers) you are no longer able to get back to LAX on a direct flight, because of the 1 transcon only rule. In the second case (SYD-JFK direct), you can.