Many people find that they're great at GCSE's but terrible at a levels for a subject or vice versa. An A* is no guarantee of a good mark in a levels, and an 'average' grade of a B or a C doesn't necessarily mean you'll get a low grade in that subject for AS.
In addition, I know many people, myself included who didn't try their best or anywhere near for gcse's, giving them lower scores than if they had tried. When it comes to a levels, they may start working extra hard, giving it their all and doing very well.
Finally, although what I'll say now isn't concrete, I've found that for certain subjects getting an A* at gcse wasn't really that challenging or difficult (for lack of a better word). And this kind of links to my second point, someone may not put much effort at all into a gcse, get an A/B/C/whatever and then go on to a levels and turn the trend of 'poor' gcse's=poor a levels upside down.