I finally figured that out myself, but that's an insane policy. You can have far more variations using alpha/numberic passwords that just numeric. And they should clearly say that the PassWORD cannot use any letters, only numbers.

The whole website is like it's in a third world country. Oh... wait....

Try to sign in to check your flight or maybe change your seat. You can't use you're Confirmation number. You have to use your Ticket Number. Who makes a note of their ticket number when they make a reservation. I had to go back through a few months of emails to find the original UA receipt. Was actually surprised that the TK site accepted a UA ticket number.

The site is certainlly not very intuitive or user friendly.