I have just logged in from my home computer, and tried to make a post and was given the note that I most recently posted from my current IP address less than 360 seconds ago even though I just got home.
Have anyone experienced this before?
Same here.
I don't understand why a login will reset the timer.

Use the time after login to read forum posts before replying. It is issue only if you visit and read the forum logged off an login only if you want make a post...