I had missed it. Thanks
Well, sure it's easy if we ignore the docs 
But, we can't well explain what you have seen from the published material.
Your test doesn't prove that 24H is the bucket refill cutoff. You only tested non-ARI renewal at 24H and rapid renewals (1min) so it could be anywhere between that. Or, it could be something entirely different as a limiting factor. What you proved is there is some rate limit on a non-ARI renewal.
Your cert uses a single IPv6 address as the identifier (not mixed with a domain name) so perhaps that is a factor. IP address certs are fairly new so you may be running into an LE bug.
We'll likely need to wait for @aarongable to clarify or someone to review the Let's Encrypt Boulder code.