probably cuz alu is lighter? from most of the cheaper CF seatpost the weight difference between them and alu. seatpost is not that far apart. i'm going try the american seatpost first when i get it... i want to compare it between my weyless and that. i doubt i will find much difference... but if i have extra cash left over for the build.. i might pick up the supergo aero seatpost.. just cus it looks cool.. lol