Siphon vs gravity feed fuel tanks

[overshot]

i don't know about scientific. but the advantage of top draw, which manufactures started going to in the 80's, is you don't have to worry about getting a fuel line jerked off by an alligator. some truck owners, that started having tank equalization problems, would shut off the return line to one tank and run a crossover line between the 2 tanks so they would equalize. which may be what you are seeing. or it just may be the old style bottom draw-top return.