The credit card company determines the APR range for each credit product it offers. Credit card issuers often set different rates that apply to different types of transactions and different circumstances. For example, various APRs for a promotional purchase or introductory period, regular purchases, balance transfers and cash advances. Some card issuers also impose a higher penalty APR if you miss a payment.
A fixed APR, like the interest rate on an installment loan, does not change. Variable APRs are more common for credit cards. This rate is typically tied to the U.S. Prime rate (the overnight rate banks charge each other). For instance, if the Prime rate is 3.75% and the bank adds a margin range of 5.74% to 8.74% to the variable purchase APR, the variable purchase APR range would be 9.49% to 12.49%. As the Prime rate fluctuates so does the APR on your card.
The APR set for your account is based on your creditworthiness and other factors (which includes your credit rating/credit score). The better your credit score, the better your chances to be eligible for a lower APR (within the range set for the card). On the flip side, a lower credit score typically means a higher rate.