HELLO Tung Tran Van…
well i think it has to do with the signal iterations…i suggest u to increase the signal iterations from 1 to two or three in the global layout window and then when you again check for the OSNR , increase the signal index and see where your OSNR is better..For changing this signal index , i suggest you to follow the link below.. I hope it is helpful to u..
optiwave.com/resources/applications-resources/lesson-5-bidirectional-simulation-working-with-multiple-iterations/
Thanks and regards