Well that's also what I used to believe, kinda makes sense. But drawing from my own anecdotal experience I had to conclude that's not the case. Maybe I'm wrong, and it's just VERY bad at it, or it doesn't work where I live.
Well you can set a departure time and it gives "Typically ..." so it must have some information like that. And their Waze gives a graph of journey times by time of departure.