Answer - when it is purchased with two 1-way fares. I ran into this problem last week. Purchased a "roundtrip" ticket from SFO-LAX-SFO about 10 days before travel. Flew from SFO to LAX on Monday, 1/13 with the return scheduled for Friday, 1/17. On Wednesday, 1/15 found out I needed to go back on Thursday 1/16, so I called AA with the idea of paying the $100 change fee and switching my return from Friday to Thursday.

Can't do it, they say. Your ticket was purchased as two 1-way 7-day advance purchase fares, so each leg is treated separately. Since you don't have 7 days before the return flight, you need to buy a new 1-way ticket to go back on Thursday. Fortunately, the new 1-way ticket was only $165, so it only cost an extra $65, but it could have been a lot more in a different situation.