Did you do a partial boil by any chance? I see the recipe is for a full boil, but I'm just curious because adding top-up water after the boil is easily the most common reason for an off OG reading in an extract recipe. In that case, it's just a false reading - those sugars are there, but they weren't mixed evenly throughout the wort.
If you did a full boil instead of a partial, double check whether you really ended up with 3.25 gallons of wort - if you have more than that, your extract is more diluted and you'll get a lower gravity reading.
If you've got the correct volume, think back about whether you really got all the extract or if there was some that you couldn't get out of the can. Also, 1 lb of corn sugar is listed in the recipe - did you remember to add it, or is it possible it was forgotten?
If you're sure all that is in order, calibrate your hydrometer - make sure it reads 1.000 in 60 degree distilled water.