The web site should recognize any vin number, make sure you didn't make a mistake. Unless you aren't in the USA and may need to hit a different official site. However, IIRC the A model was the original 900 Ninja back in the 80's, the B model was the porker they put out in the 90's with a motor that was based on a bored and stroked ZX7R (you have a ZX900-B2), and the C model would be the one after that weighed only slightly more than the ZX6R and looked similar as well. The number indicates what year of that particular model, for example the original 900 Ninja would be a ZX900-A1...and your bike would be the second year of the B model.
Rarely is the question stupid, but sometimes the answer is
you need to run everything you read online through your own personal BS meter to determine if it makes sense to you.