I just ran one of my sites and it shows "Daily Ad Budget: $4.94 - $45.93"
The real daily spend for that one is around $650.
So it's not always the case of them getting it "pretty close".
They are good at getting basic keywords, but they can't possibly scrape ALL results for ALL sites in ALL niches.
I was worried about them a while back, but not so much now.
They are really good for brainstorming if you are out of ideas, but don't rely on the numbers.
As a side note (and don't take it personally), but if your real stats are close to what they are showing, that means you are not thinking outside the box enough with your keyword selection. In other words, you are only covering what most of the people in your niche are covering.