I think a somewhat dreamy picture is being painted. At least 10% of students get 2:2/3rds - that's over 300 people at Oxford each year. Hardly negligible! Hardly likely to all be entirely due to illness or laziness. Fact is the admissions system, good though it is, is not perfect. Mistakes are made, especially when some subjects have very fewer applicants per place (barely 2:1 for Classics, BioChem etc).
Sometimes people aren't good enough and when that's happen it's desperately sad and not to be dismissed. It's also why people shouldn't look for easy ways into Oxford all the time - the fact is if you do cheat the system, it'll be you who suffers as you struggle to cope.