Position in an event - Not a race!

I’m a newbie to this zwift malarky - having fun! Today I did the KISS Winter Sportive. I was a little late (30 seconds) in getting started and never really found a group. I did ride with various folks along the way. I tried out a bit of drafting, but generally kept my watts around tempo / low threshold, so while it was good hooking up with folks, I found I typically rode by them eventually.

Anyway - my question is about position in the event. I finished around 100 out of 330. In zwift power, I appear to have ranked 9th in the category. I know that is just a subset of those on the ride - but - there were riders ahead of me on the ranking, whom I passed during the ride. I know it’s not a race, so doesn’t matter - but just curious how this is worked out - if indeed it is a science?!?