Synthethic Eurchf Vs..

can you resolve this equivalency for me:
in the past on 1 day:
eurus=1.36761
uschf=1.19977
eurchf=1.64069
synthetic eurchf=eurusXuschf=(1.3676x1.19977)=1.64081

2 days later:
eurus=1.36196
uschf=1.21046
eurchf=1.64880
synthetic eurchf=1.64880

if i went long the synthetic at 1.64081 and exited at
the synthetic at 1.64880 my profit would be
$642.73 ( 1.64880-1.64081=$799x(1/1.21046)=$660.

can I duplicate the synthetic's profit by going long
eurus and long uschf (100000lot) correct?
Calculations dont match.. Please Correct my work. Thanks

You best read this thread

http://www.forexfactory.com/showthread.php?t=64423