The gentlemen above have very kindky privided you with the reasons for the difference, but I'm going to guess that you don't know why you need this information (I certainly didn't have a clue when I first got a PPL). So I'll try answer it before you ask. The most important thing to realise is that on very cold days, your altimeter will over-read. You will be physically lower than your altimeter says. As most PPL flying is VFR over flattish ground, it shouldn't matter too much because you can visually avoid obstructions. But as soon as cloud gets in the way or you start messing around with mountains or tall structures, this becomes rather important. Not knowing your true altitude could result in you unintentionally too close (or worse) for comfort.
PM