Originally Posted by Revvy
It's a pretty common issue for ANYONE topping off with water in the fermenter (and that includes partial mashes, extract or all grain revcipes) to have an error in reading the OG...In fact, it is actually nearly impossible to mix the wort and the top off water in a way to get an accurate OG reading...
Brewers get a low reading if they get more of the top off water than the wort, conversely they get a higher number if they grabbed more of the extract than the top off water in their sample.
I agree with this phenomena. I had this happen on my first batch, and thought better and as Revvy said, the proof was in the pudding. It all did its thing and I had a strong porter as I had hoped for. I would say don't sweat it, you probably took a poor reading.