Are you talking about UMS or raw marks?
If UMS, then no, the boudary for an A is ALWAYS 80%.
If you're talking about raw marks, then yes it can vary. I wouldn't call it tampering though. It's taking into account how well everybody did sitting the paper, whther in England, India, Australia, etc.
If they didn't do this, you might have some years where no one comes out with higher than a C, because the paper was difficult.