Website differences are a continued bane of a web developer/designer - this in most cases is due to the different rendering engines used

The Five most common rendering engines are -
Trident developer by Microsoft used in IE, Windows mobile and AOL browsers,
Gecko (Mozilla) - Firefox and Firefox OS,
Presto (Opera) - was used by Opera but now replaced by Blink but is still commonly used,
Webkit (Apple & Google) - used in Safari, Android and Web OS to name a few
Blink - (Google) - an off shoot of webkit and developed soley by Google - used in the Chrome browser