There are probably two key factors behind the regular improvement in results:
1) There is a bigger coursework element involved in exams these days. Coursework stretches pupils in different ways to the old, cram a full two year's study into two exams method, but it is, inevitably easier not to cock up.
2) There is a larger base of past papers to work through and teachers are better placed to second guess likely exam questions. There is a finite number of questions that can be asked on any subject and pupils are being better prepared for the variety of topics that might be thrown up.
Therefore, exams are easier to pass, but this is not necessarily to the detrement of school standards. The top achievers in our generation are probably less intellectual than their predecessors, but perhaps better placed to progress in a modern world.