valve talked about it on a reddit thread a while back, i cba to search for it
but if you think about it it makes sense, thats why if your rank expires you dont have access to overwatch anymore
read the "Aren’t mediocre players going to falsely convict good players?"session on the official faq blog.counter-strike.net/index.php/overwa..
even if a gold nova watches a global elite player, his veredict wont count if his score is low anyways
rank doesn't affect overwatchers "score", it's just based on their effectiveness
an MG1 overwatches GEs too, and yes MG1's vote can have bigger impact than GE's vote, if MG1 was more often correct previously