Interest is calculated in one of three ways: daily, monthly or every six months. The bank pays interest to accounts either once a month or once every six months.
Because of these variables, and other factors, the amount you earn on your savings can fluctuate.
Factors that determine the dollar yield on an account:
Current interest rates
How interest is calculated on an account (ie daily, monthly or every six months)
When interest is paid on an account (either once a month or once every six months)
The following factors reduce money earned and can even turn it into a loss
Fees, charges and penalties. These are usually based on minimum balance requirements or transaction fees.
The maintenance of a certain balance in the account. Many accounts don't pay any interest at all unless a certain minimum balance is achieved and maintained for a set period of time. Generally speaking, the higher the balance, the higher the rate earned.

