Registering the computer doesn't eliminate all the safeguards built into financial sites. I never register my computer, because I want my financial sites to always ask security questions. That provides extra protection in case my MBP is ever stolen.
Usually this info is saved in a cookie the first time you go through the bank authentication routine. Are you allowing cookie storage or have you deleted cookies since the first time you signed in with the bank?
If your IP address changes, they will usually ask you to answer a security question again. For example, if you log in at home and answer a question, then log in at work/school you will have to answer a question again. Your IP address can also change if your ISP decides to change, which can happen with most residential plans.
You know I noticed something after I tried logging into my bank, recently it has asked me for my pin and security question everytime. I would rather be safe than sorry, so it's no big deal. I can't have everyone trying to access my millions of pennies.