I want to point out that I do not regularly look at my own profile, so my likely, I was promoted on June 30July 1 (when I posted after the new activity period), but did not notice until yesterday.
Ah, that might explain the problem here. I was always under the impression that the transition from Hero-->Legendary happened on the day of the activity period change. I'm pretty sure that was the case when I made it to Legendary, because I was anticipating it for weeks on end (I finally made it when I had an activity of something like 1064).
If you have the same number of posts that you do activity, and you have made an average of less than 14 posts per activity period, you could potentially rank up on a day other than the start of the activity period. The activity threshold to rank up to legendary can be anything between 775 and 1030, and is not necessarily a number divisible by 41.