I have been banging my head against this for a few days and finally I purchased two one ways tickets as follows:

8/25 SAN-ORD $92 + $17 = $109
9/02 ORD-SAN $129 + $10 = $139

Total = $248

Same flights booked as round trip are $314 + $20 = $334

Anyone know why this is the case. Also just noticed the inconsistency in tax charged...