Sherpa pet carriers / United Airlines / Help! Confused....!?

I'm flying from Narita Japan to San Francisco in May and I'm bringing my cat with me. I'm going to carry him on in a soft Sherpa bag that says it's "Airline Approved".
(I'm flying United) Just to be sure, I called the customer service number for United Airlines and asked them about any size / weight restrictions for my cat and also what the maximum dimensions of the pet carrier were. I was told that my cat's size and weight were OK and that the maximum allowable size of the pet carrier has to be:
17 inches long
10 inches wide
10 inches high
So I go to the Sherpa web site:http://www.sherpapet.com/products/detail...

I see that the carrier that matches my cat's weight is the medium sized carrier, but it's too big! It's:

18 inches long
10.5 inches high
11 inches wide

This makes no sense to me. How can Sherpa be Airline approved for pets when the carriers don't conform to the allowable dimensions??? What if we get turned away?

I think each airline has slightly different kennel requirements. I would get one that adhears to their dimensions so that you have no chance of missing your flight due to your kennel being too large.
You might also call back again and see if a different person gives you the same dimensions or not.