The apparent attenuation is higher than actual attenuation because the alcohol produced during fermentation is lighter than water. If you wanted to obtain the actual attenuation the alcohol has to be removed by heat and replaced by water. Sounds fun huh?
Only the big-boys go to so much trouble to find the "actual" attenuation. For homebrewing just worry about the apparent attenuation.
The apparent attenuation you get depends on the types of grain or extract you used, (if brewing an all-grain) the mash temperature, and even the strain of yeast you used. Apparent attenuation generally will range from 65 to 80%.
Yes, beers with a higher apparent attenuation will result in a drier, thinner, higher alcohol brew and a lower apparent attenuation are generally sweeter, fuller-bodied, and lower in alcohol.
Or better yet... don't worry about it much at all. Don't let all the "ins-and-outs" keep you from enjoying brewing beer at home.