Fact of life I am afraid, air contains moisture, warm fuel returned to the tank actually helps the moisture on the inside of the tank form. Nowadays less of a problem with plastic fuel tanks, the water is still there but simply stays with the fuel and gets caught in the filter or water trap. Year ago it was a big problem when driving HGV's, we actually had a water trap with a glass bowl bolted to the side of the chassis, we had to remove and drain the water out and clean pretty much on a two weekly basis.
regards
Dave