In PR G/L account pick wrong for service

Hi Gurus,

I have created contract with service and a/c assign "U". when tried to create PR against that Contract system pick up wrong G/L account.

but when trying without that contarct defsult G/L account is correct. please help me to understand what is the different between these two scenario.

and how system pick default G/L account for both cases.

thanks in Advance.

Sandip