Last week I brewed an Arrogant Bastard clone. My pre-boil gravity was off by .001 so I was pretty happy with that... But my OG (post-boil) was off by .009! I thought that because my pre-boil gravity was so close to the original (estimated), my OG will result as well in a closer number to the estimated OG.
My boil was vigorous and I boiled for 90 min (as the recipe stated) so I have no idea what could have interfered in reaching my estimated OG.
PS. My gravity measurements were taken at the same temp.