Generally I find that AS levels are too easy not to get super high marks in, if you're Oxbridge material. And even if you make a few careless mistakes you can still get 100%. Some papers have conversions such as 85% raw ----- > 100% UMS.
So while the difference is minimal between 90% and 85%, I do struggle to see why top candidates would lose that many marks in the first place, unless if they didn't work hard enough.