I ran into the same problem this weekend. http://www.flyertalk.com/forum/unite...l#post26422101

I THINK it means that the lowest fare happens to be a flexible fare, so they are default to showing "not available" in the economy - lowest column.

Horrible design, if indeed that is the way they want their website to operate.

You can use a different interface (hipmunk, google flights) to confirm there is no lower United fare available then what is shown on United.com