Show me a punter with 'accurate' [meaning exactly that and nothing less] ratings and I will show you the same man [very rich] who definitely has no need to back more than one horse per race. Why would he bother ?

The rule is always back overs and that is much harder than most punters think. There might be only one or two bets per meeting [or none] where the odds are plainly better than your selections real chance of winning [you have information from a handicapping angle or some other source not accessed by 'the public' and they don't miss much].
To find 2 horses in the same race that are genuine overs from your short list is la la land stuff. It happens so rarely that on most race days you would never find a bet.