For those that know the fare structures better then I, could you please explain why some routes have their one way fares exactly one half of the round trip fare, while other routes have huge differences.

For example:

IAH - LAX shows a R/T of $290 on 8/18 return on 8/21. The one way fares for the same dates are $145 and $145...exactly 1/2 of the R/T fare.

However, IAH - JAX shows a R/T fare of $233 in September but the one way fares for the same dates are $302 and $302....$371 higher than the R/T fare.

Only reason I consider booking two one-way tickets (assuming it equals the R/T fare) is to get the extra 1000 mile online purchase.