Pet Carriers For Flying Sherpa Pet Carriers / United Airlines / Help! Confused....!?

Sherpa pet carriers / United Airlines / Help! Confused....!? - pet carriers for flying

I fly to Narita in Japan in San Francisco in May and I take my cat. I'll take in a soft bag Sherpa said that approval, "Major Airline.
(I fly United) just to be sure, I called the service number of United Airlines and asked about any size and weight restrictions for my cat and also what the maximum size of the company's Supply Company. I was told my cat, the size and weight have is good and that the maximum size of the pet carrier must:
17 cm long
10 cm
10 cm high
Then I go to the website Sherpa:%%% 20bag http://www.sherpapet.com/products/detail.php?proddetail_name=Original 20Deluxe AE & proddetail_post = Y

I now see that the weight is my cat is a medium-sized businesses, but it's too big! East:

18 cm
10.5 cm high
11 cm wide

It makes no sense to me. How can it be safe for the animals Sherpa aircraft, if companies are not allowed to keep their size?? And when you turned around?