Partly because the vast majority of people who got them had sat the Oxford entrance exams, where the questions were tougher then A-levels. This is how I got in. My college was the first, in ‘modern’ times, to offer entrance without the exam. But they insisted that if you wanted to apply that way you had to put the college top of your preference list (sneaky) and also come up for additional interviewing (you think exams are pressured !). And then they made hardly any offers this way - just a handful out of a yearly student intake of nearly 100.
VB