I have two credit cards from the same bank and I noticed that they were almost all the same number. That seemed strange to me since I got them at very different times.
Turns out that you can tell a lot just from your credit card number. For example, the first number tells you what type of company issued it: a bank, an airline or an oil company. The first six numbers will tell you exactly which company issued it (if you had the master sheet of companies.) You can read all about your credit card number in Anatomy of Credit Card Numbers.