A confounding variable is an extraneous variable that is related to your independent variable and might affect your dependent variable. In an ideal study, there will be no confounding variables.
For example, student accounts have a limit of $100 limit per day, while non-business accounts usually are limited to $500 withdrawals a day. For business accounts, the bank offers $1000 withdrawals in one day. Chase also offers customers a withdrawal limit of $1000 every 24 hours.
For example, if your daily withdrawal limit is up to $600, but you are at a machine that has a limit of $400, then you will only be able to withdrawal $400. In general, for ATM limits you are looking at anywhere from $300-$1,000 with $600 being the average.