Have noticed a weird case when trying to reserve tickets.

I am booking IAH-LBB-DEN-IAH. I have selected "Lowest Refundable Fare".

If I select the cheapest flight on each segment, the total price is $371 before tax. However, if I select one of the first two segments as not being the lowest priced ($415), the price then jumps to $833, with the final segments being in the same fare class as the lower price.

Seems a bit odd.