There are, broadly, four possibilities, aren't there? Unless anyone can think of a 5th.
a) Young people are getting brighter. It's astonishing that, 20 years ago, we even knew which end of a pencil to hold. And our parents' generation, phhht, amazing they even found their way to school.
b) Exams are getting easier, and it was harder to get an A twenty years ago than it is today.
c) People are taking subjects in which it is easier to get higher grades (Media, Leisure, Batik, etc) rather than proper subjects like Maths and German.
d) Teaching is more carefully targeted towards the passing of exams, rather than the imparting of knowledge and the acquisition of tools to analyse it.
Well, a passing acquaintance with teenagers would make (a) hard to espouse without guffawing uncontrollably.
As for (b), it's hard to prove without a control experiment - which I'd quite like to see. (Not fair just to give a current paper to a 38-year-old - they'd have to have had the same teaching. Ditto giving a 1987 O-level to an 18-year-old.)
I tend to go with a (d), plus a bit of (c), and DW, who is a secondary school teacher, concurs. There is a a lot of pressure on teachers these days to improve results - league table obsession has come in during the years since we thirtysomethings left school, and every point counts.