Issue with rate limit setup

This is regarding rate limit issue in shield waf.

1. If rate limit is 10r/s and we send 10 request in a second on what order the requestes will be counted in
2. What will the sequence of requests landing in
3. Ideally all requests should be passing what could be the reason if an intermittent request is failing due to rate limit
4. If 12requests are passed ideally last two should be failed ...but if the 6th request is failing and last 2 are passing what could be the reason ...