The way I heard that scoring works is that your performance on a champion is compared to other people (of similar skill)'s performance on the champion.
I'm no expert by any means, but if you have more assists that kills on an ADC, that won't be equal to other ADC's performance because they are probably getting more kills. 53% kill participation is great, but you should try to kill secure in those cases.
I don't know how vision score plays into ADC performance because jungle and supp are usually leaders in vision, but if you got a "decent" grade on a test, that doesn't mean that you aced it, that's probably a B. An S or higher belongs to the best players on that champion (of your skill) so if you believe that vision score is a big factor, make sure to get a lot of it. Get control wards on EVERY BACK. As ADC, convert to blue ward around level 9. It's easier since you aren't RISKING a death with a facecheck (since you have range), and with that safety and range, you can ward more frequently.
You'll get it eventually.