A portal implies that you have a one to many relationship. One record in your layout's found set linked to many records shown in the portal. When you put such a field from the related table on your layout, it only refers to data in the first related record unless it uses an aggregate function like sum or a summary field--which then computes data from all related records. When you put this field inside the portal row, you see it evaluated based on the data from each individual record in the portal.
So the first thing that I'd check is whether the results of the first portal row (if the portal is not sorted), match the results you see when the field is placed outside of the portal.