GeeCee's Not-Entirely-Original Miscellany

Why, exactly, is it less expensive to buy two one-way tickets from Dulles to Oakland and back ($149.80 each) than it is to buy a single round-trip ticket for the same flights ($380)? And why does Travelocity want to charge me $130 in change fees plus ticket price differential when Delta themselves will charge me much less than that (and in both cases, of course, declining to give me additional credit if the new price is lower than the old). Do I shell out for the change fees at Travelocity, or do I just cancel the return leg, use that ticket later, buy another ticket on JetBlue for not that much more and get a much better departure time on Friday (11:45 pm rather than 1:10 pm) and a cushier overnight flight? Must decide tonight.