A question about library cards

I received this question the other day and I have a guess but not an answer. Can anyone help? If so, please post it as a comment. Thanks.

I have a random, totally irrelevant question that I think you would be the perfect person to answer. I was looking through my library card collection (I have about 8), and I noticed that all the card numbers start with the number 2. I was wondering if you could look through your collection (which is much larger than mine) and see if you notice the same trend. I was also wondering if you have any information as to why this is so. --Bud

(My guess is that it has something to do with the company that the cards/barcodes come from. Many of mine start with a "2" but others start with a "4" and cards from the Denver Public Library start with a "D".