Reason for disagreement in bank book and cash book?

Are you sure there isn't a transaction still pending? Did you remember to mark off all of your fees? If it is an interest-bearing account, it's possible that the bank calculated the interest at a different time than you did, when the amount in the account was different. And lastly, it's also possible that a deal the bank had been offering you, or a program which you had enrolled in for no-fee at the time, has expired or has had a fee added.

You can usually go into the bank and get a record of the last couple of days. Carefully compare your statement to your records and you should be able to track down the discrepancy. If it is an expired deal or a new fee, you can usually politely ask someone about having it removed and the service cancelled. If it is a recent change, they will usually help you.

No matter what, the best person to talk to about it is someone at your bank. If you go in with a positive attitude, they will usually do whatever they can to correct the mistake. If you go in angry, or yell or accuse anyone, they won't do much more than follow the exact letter of any agreements you've signed.

You could have a check that was written, that has not cleared the bank.
You could have made a deposit that has not cleared the bank.

You could have a CD that matured that has not been post on the books.

You could have a fee or interest that was added or subtracted from your bank account that has not been posted on your books.

You could have an error in books like recording the wrong amount on a check or a payment from a customer, or you could have made a math error.

There are a lot of reasons your book balance and your bank balance are not in agreement. When you receive your bank statement you are required to do a bank reconciliation to bring your book balance and your bank balance into agreement.