I understand, I have the H6 rear rotors on my WRX. I know exactly what you're talking about.
I'm unclear as to why this is confusing. You know that these are the stock rear brakes on the Legacy H6, right? As such an aftermarket rotor designed to fit the Legacy H6 will fit the H6. I don't understand the confusion. It's not that everything 'just happens to fit' as you say. The system was DESIGNED as a rear brake setup by Subaru for the Legacy H6. Since the H6 shares the rear hub assembly with all other current Subarus with discs in the rear, it bolts right up to the WRX.
Interesting to see a vendor package it and sell it as a kit...especially at such a tremendous markup
That's about $100 more than I paid from Irvine Subaru...and we all thought dealers marked up prices astronomically.
If you buy the stock H6 caliper brackets from your Subaru dealer (26625AE000) and DBA 657 it WILL fit. No question.