Interesting numbers becareful. There was a theory somewhere that suggested punters do the form in reverse race order. So R8 first then R7 etc. as many doing the form from 1->8 speed up their analysis towards the latter races and thus not as thorough, hence more false favorites.
Flipside is that most of the earlier races contain 2yos,hurdles,fewer starters which limit chances, whilst at the other end larger fields in restricted company, fillies & mares etc., so would probably support the descending %.