I too find the reasoning odd. I work extensively with times in my day job. If a time period starts at 2021-06-10Z22:23:00 and finishes at 2021-06-10Z22:23:01 then that represents one second. For it to represent two seconds, the end time would be 2021-06-10Z22:23:01.999... which is the same as 2021-06-10Z22:23:02. If the RFC does not explicitely state that this is the desired interpretation, I find it odd to assume this is the case.