Correct. Efficiency is how much sugar you are extracting from grain in a mash. Attenuation is how much of those sugars are being converted into alcohol by the yeast.
(OG-FG)/OG=% of attenuation. In other words. if you start with 1.050 and you finish with 1.010 that is 80% attenuation. With most American yeasts you should be able to get 75%. I don't think that 5 points is all that much though. Your hyrdometer could be off. You probably couldn't taste much difference. if you were 10 points off then i'd start to worry.
If you are having issues getting your FG low enough then I'd suggest using the pitching rate calculator http://www.mrmalty.com/calc/calc.html
to make sure you are pitching the correct amount of yeast.