When a functional field is computed?

In OpenERP 6.1 I have a functional field in product that is displayed in it's tree view. When I see the tree view it takes a lot of time to display since it computes all the fields for all the products in the view and this is quite anoying.

Because of this I decided to put "store=True" on the function field so it will not recompute the values when I open the tree view. I also check that the values are stored in the database. Still, when I open the product tree view the values are recomputed once again. So I want to know when the function fields are computed in general? and when are computed with the "store=True" option.