IIRC, its because of the way the tank is shaped. Bigger at the top, and smaller at the bottom. therefore the gas level goes down faster at the bottom.

Someone correct me if Im wrong.

-Ken V. 1998.5 SE Praire Tan Zetec ATX psycho_bass@hotmail.com Roush springs Roush rear sway bar BAT struts 17" Millie Miglia HT3 and a ton of subtle asthetic mods