Unique visitors per day, week, month, year

I saw a couple of posts about this topic in here but no one answered it exactly.

How are the unique visitors per period are counted?

per day - the real uniqe visitors a day
per week - it’s not the sum of all daily uniques, it’s not the average

example:

We have piwik running in a LAN with a limited number of static IP-Adresses.

Daily uniques are ok. But the week/month/year count is far bigger, than possible.

Does it count the returning unique visitors? But how exactly?

If i query the database and convert the “location_ip” and make a “group by” the number is e.g. 1000 but the yearly uniques are 15000. But as I mentioned it is not just the sum of daily uniques, because that would be 30000. So how is that calculation performed?

PS: We’re using piwik 1.0 until they fixed the visitors bug in 1.1x.

Ok what I knew now is, that the visitor_idcookie is counted and that value is e.g. 15000 but the location_ip value is 2000 - How can that happen? The users browsing with the same Browser, OS, IP, PC. Cookies are stored in their browsers and no policy is removing them on restart or somethin else… strange.

Hmm i found visitor_idcookie for all same settings (resolution, browser aso.) for ONE day? The SQL row is equal, except the exact hour, minute and of course the visitor_idcookie