Why does....

...this happen?
Sometimes when I click on a thread I've been following, it'll go to where I'd last read to, other times, it just goes to the start of the thread, and I have to scroll through pages to find where I'd left off.
Its a cool feature, saves time, but doesn't always work.
I checked the tutorials, but couldn't find anything relating to this.
I did think maybe it was because some threads I've been following may have been before the change to V7, but this isn't the case.

 

If you click the red dot at the left of the thread title it takes you to the new first post that you haven't seen.

 

Thanks, that'll be even faster.