Every item measured against what its own price is worth, not against the whole shop. A 6400 item wins more than an 800 one because you only buy it in a match that already went long — so the number here is how far each item beats the others that cost the same. Each purchase also counts against how often its hero wins: an item only strong heroes buy doesn't get their win rate for free.

Each band is measured on its own matches, not on the ranks above it.