I am nearly at 1000 activity but I am still a Hero level member.
I guess because I do not post very much.
After you posted this comment here, you have already became a Legendary member, is someone make this happen(not random)?
No, their activity could be higher but is limited by the number of posts. It will increase with every post until the limit is hit. Because the user often posts less than 14 posts in 14 days, this limit was actually above legendary max (1030) for quite some time.