I don't get it. If it's not stored in the database, where is it stored?

it's not getting stored in DB. it's calculated on runtime.

Its pretty annoying though - from a user perspective it just looks like openerp doesnt work.. Why dont they hide the fields from the advanced search if it doesnt do anything??

@Timothy, I too thought it was annoying. This is especially true since you can easily do things like add dynamic sorting to your tables after these fields are calculated. Use something like DataTables in jQuery just for a quick example. They could have also written their own for that matter if they are going to provide the options but not the functionality.