What does the Column Sum calculate?

Hi,

the column sum function isn't behavin like I expected.. at all and hopefully someone has an explanation or even a solution for me.

Following case:

I have an page area name in on eVar

The product name in another one populate by prod syntax

If I want to know how much products were looked at and create a break-down with

> Page Area

>> Product Name

With Prod Views as metric each product shows the number of its own views and the total is lower because it counts unique 

So it may look like

> My Products  70 000

>> Prod A 50 000

>> Prod B 20 000

>> Prod C 20 000

>> Prod D 10 000

If I want a total sum of all the prod views in "My Products" (100 000) I assumed column sum now would do the trick because it sums up all the metic of a dimension item. (Reference basic functions | Adobe Analytics)

When I create a calc metric with "COLUMN SUM (Product Views)" the result would be something like 90 000 for the example above. If I dowload all the items and add them up the sum is higher than the column sum. 

Did anyone experience the same and/or has any explanation for this?
Maybe there is some obscure logic behind this which I simply don't see?

Thanks and best regards 

Uwe