It is probably the quick reply textarea. you can control the width of the text area (colums) from your control panel. I found the bug with the "email notification of new replies" not showing. So that may have made you screen wider?
Msg#: 4175241 posted 2:09 am on Jul 24, 2010 (gmt 0)
Followup to Brett and kiddies... I think I have discovered WHY the right scroll sometimes comes up (and thanks for the tip on resizing textarea as that helped, too)...
The scroll seems to occur when the CODE tag is used in posts... and that code extends beyond the norm. Not sure how one can deal with that in presentation layout but does make perfect sense as to why some posts are WIDE and the others are not.