What is your native language? Maybe someone can help translate what you're trying to say. I don't understand your issue.
That number means "the highest number of people online
at the same time during the day".
The "at the same time" means... I think is the same time as the "show members online in the last xx minutes".
So, explaining the reason it is the same at the end of the day, it's because if for example at lunch time 50 people come together at your forum, the number goes up to 50, but if during the rest of the day, the number of people connected to your forum
at the same time is never higher than 50, the number doesn't change.
I hope it makes sense, I'm not that good at explaining that kind of things... :-[
Provided this is the problem, of course. ;)