The relationship between the two
APR is the quoted annual rate, stated before anything inside the year is compounded. APY is the effective annual yield: what that rate actually earns once interest is credited and then earns interest on itself. The two numbers agree only when interest compounds once a year, and the gap widens with every additional compounding date, because each credit lands on a balance that has already grown a little.
Both directions on this page run on that single definition, so the numbers you get are inverses of each other rather than two independent approximations. One caveat about the word APR: on a loan, the disclosed APR can include fees, and one built that way may sit above an APY calculated from the same note rate. The clean one-way relationship - APY at or above APR - holds for fee-free deposit accounts, which is the case this page prices.