Fermentability of Extracts, Maris Otter and Beersmith

I'm trying to understand my ingredients better...

I used 6lb of Maris Otter Extract in my currently-not-doing-great batch of Boddignton's clone. My OG was only 1.041. Beersmith tells me that I should have expected 1.046. Now, I might have messed up and steeped my specialty grains wrong, but 5 points seems a lot for so few grains (about 13 oz total). Or did I just screw up somehow, and miss my expected OG? I don't see how though, unless it was to do with a late extract addition (3 lb for 60 min; 3 lb with 15 min left)?

I also read in Papazian's book last night that different extracts can have different amounts of fermentables - some might start low and finish high.

So, it looks like I'm going to get a bit of a wimpy beer. Not too worried about that, but I do want to know if that is what I should have expected, so next time I use this extract, I'll know if I need to up the amount or change the way I do things.

I'm guessing the numbers in Beersmith are wrong - I just made a new entry for it, using the same figures as for a Pale Liquid Extract. The values I had in Beersmith are for a Potential of 1.035 OG and the "Dry Yield Fine Grain number" is currently 75%. If I bring the OG number down to about 1.031, then it looks like my measured OG is close to the predicted.

Any suggestions???

You reall aren't that far off, it could just be some other variable threw off your reading - temperature, top off water not being thoroughly mixed, etc

I had shaken it well, to get some air in, so I don't think it was top off water diluting it. I might not have corrected for temp, so that might add a couple of points, true. It was under 80, but probably over 70F when I took the hydro reading. So that would put me 3 points shy still...