The pH of the food is different from the pH of the cat's urine after eating that particular food. The words to explain that are escaping me now.
Let me try again...
Some companies have a target urine ph that they claim their food will produce. A common range may be 6.2-6.4 which is optimum.
The pH of the food isn't worth worrying about, IMO. It's the pH of kitty's urine that is of more concern. The relationship between the food's pH and the urine's pH isn't parallel, from what I have read.
The 5.5 that Felidae states..is, like you said - the pH of the food. That can't (and shouldn't be) what the urine pH would be after eating that food. I hope that makes sense.