i am afraid you are wrong on this matter
if you put a fort on a resource you don't get any benefit from that resource, if you improve the resource outside of the city cross you at least get some of the benefits of that resource, eg. iron+mine=your civ gets access to iron
prior to BTS i have never seen a AI build a fort, now they are building them where:
a. it it is pointless building a fort
b. on a resource