How? Many routes have YUP fares that are in many cases lower than the actual Y fares. Not all routes - and I didn't say all. All I was saying is why someone may choose a Y fare over a B fare.

For example: I did a quick BOS-LAX fare check on AA. There are the following Y fares:
YUP at $1738 with a 7 day advance requirement
Y26 at $1908
YUP at $2008
Y at $3346