Browsers (Google Chrome, Mozilla Firefox, Internet Explorer, Safari and other) are updated once every one or two weeks. Similar situation is with server and website software updates - PHP, MySQL, jQuerry, etc.
Moreover, integrated online payment and shipping systems (Moneris, PayPal, Interac, Sagepay, CanadaPost, FedEx, UPS, Purolator, etc.) carry out their own upgrades regularly, what influence website’s functionality too.