The difference in numbers means that what you made weighs less per unit (ounce, cup or gallon) than the directions suggest.
The abstract is that you did not get as much of the sugars from the grains of the partial mash as ABH did. I may also mean that your measurement was not accurate and there are still issues with how you are mixing and taking a sample.
I do not measure any of my worts for the same reason. I cannot get a consistant way to get a SG to worry about. If most of your fermentables come from DME or LME, you will be dang close to getting the most of the sugars. If not, you can always try again, as you have still made beer, and it will still be good.
If you are not growing your own 6th generation barley and hops, you're not *really* homebrewing.
Fermenting: Real Ale, Extract Lager (with WLP830), India Pale Ale
Drinking: Pale Ale from the keg