APR=Annual Percentage Rate
Which is different if calculated for a month. That is you have to divide by 12 to calculate monthly APR.
As, APR does not compare loans for different lengths of time, because in every month or between the period or cycle for which APR is calculated, the total credit and debit in your account may Alter .
So, for different intervals or we can call duration of time for which APR is calculated your account balance may increase and decrease due to purchase or Paying loan or Depositing Checks.
Also, the reason may be that , APR differs for different amount of value of Loan or Debiting for a certain duration, as well as the period for which money is taken, the APR fluctuates.
That's why APR is not generally used to compare loans for different lengths of time.