WTF? How is google analytics counting users?

Hi guys,

I have problem . When i choose ANY month from google analytics i see on the dashboard number of unique visitors - 320k .

But it was strange because i knew that our website got atleast 20k unique visitors a day - so lets say it should be somewhere between 500-600k (this month)

When i choose to see graph divided for weeks and put them into calculator it shows like 450k unique visitors.

When i choose ANY month there is alteast 100k more visitors according to graph than on analytics dashboard number.

Am I retarded?

I can't understand the problem . But same case for me. I know google analytic counts robots as a visitor. It may be the cause.