I am not 100% sure, but this is what I heard.
The clock only ticks away for when you are an active user. So, if you keep the page open and do nothing, then the clock will not tick away. But, if you are clicking on topics, replying to messages or just being active, then the clock will continue to count.
So, when you refreshed your page, it didn't change because you were not doing anything. It is all about being active, not how long you are on-line. There is a difference.
Hopefully, that could help you out and answer your question.