Original gravity lower than expected
I'm new to homebrewing. I just brewed my first batch today! I went with Zainasheff's Belgian Pale Ale (Antwerp Afternoon) from Brewing Classic Styles. I halved the recipe (down to 2.75 gallons) and converted it to a partial boil using Palmer's directions on converting to a partial boil and by halving the fermentables.
Everything seemed to have gone well, but when I measured the OG (regular hydrometer) I got 1.048 (at 68F). Zainasheff's expected OG is 1.052 and Beer Alchemy's is a whopping 1.061! For the original full boil 5.5 gallon recipe, Beer Alchemy gives me an expected OG of 1.055.
My fermentables were:
CaraMunich 60L 0.40 lb steeped for 30 mins
Biscuit 25L 0.12 lb steeped for 30 mins
Pilsen Light Liquid Extract 2.08 lb at start of boil
Pilsen Light Liquid Extract 2.08 lb at end of boil
I steeped the grains in a grain bag in 0.5 gallons of water heated to 168F. By the end of the 30 minutes it had cooled down to 150F.
Is this discrepancy due to measurement error, have I done something wrong, or is it both?