I’m thinking it’s trying to say:

(2/6) + (1/6) = (3/6) = (4/6) - (1/6)

But either in “colloquial English for those who want to give other people aneurysms” or “colloquial English for those trying to sound smarter but aren’t”

Basically that the degree of difference between a half and a third is the same degree of difference between a half and two thirds- and that degree of difference is “one part”.

Or ⅓ + (⅓*½) = ½

1/3 is 1/2 of 2/3