Sorry for the delay, was pulled in a different direction for bit.. 🙂
The reason is this…and maybe you have a better approach….
When a “NewForm.aspx” is used in a full window scenario (ie New button on list view) I style the form with CSS to be center position and no left hand navigation. When the form is called in a popup scenario, it is then shifted right and you need to use the scroll bar, not ideal.
If I remove CSS to hide left navigation, then it looks great in popup scenarios, but then not centered when in full screen scenario.