I have a SPD 2010 WF running on List A that creates a new item in List B. Within the 2010 "Create List item" step, I set a person field in List B to a specific user value. Once List B is created, the field value = System Account. Please do not suggest using InfoPath or 2013 WF. We use 2010 because we need to include the attachments and the "trigger Create Event" flag that has been removed from 2013. Thanks.
Probably it's because the user who runs that workflow doesn't have enough permissions, so, Application Pool account performs this.
Look at this.