I believe it has to do with available seats. If you search around a bit (or just go to Travelocity and let the low fare finder over there find you seats and then come back to United with the details for the bonus miles) then the sum of the parts will equal the whole.

The wording is As Low As, not this is the price. I have seen it actually work in reverse, where an outbound flight was as low as $149, the inbound leg was as low as $599, but the ticket was $298.

If there are any cheap seats available, then you will get the lower rate (sometimes they are not available at UAL but are at Expedia or Tavelocity, so make sure you check there for the same flights).