Well, you may have less collected wort due to: 5 oz. of hops (absorption), evaporation rate of boil that particular day (dry weather), length of boil?, etc. Also, a different grind setting could effect your mash efficiency. Would not be suprised if your LHBS has a favorite setting which is not neccessarily the 'standard' setting.
To be sure about your volume, you could always measure the circumference of the carboy and divide by 6.28 (2xPi) to get the radius. Then measure the height of the wort in the carboy and multiply it by 3.14 x radius^2 to get volume in cubic inches. Dived by 231 cu.in/gal to get gallons.
Granted your ballpark volume here would be a tad more accurate if you could subtact out the thinckness of the glass on the radius & height, but it should be pretty close. (not to mention the taper at the bottom of the carboy)..
As long as you have a tasty beer at the end, all is good!