You should be aware that the attenuation ranges given for any yeast are a range based on average wort fermentability, but what’s in an average wort?
There are many, many factors that can affect (or is it effect) attenuation and yeast is only half of the equation. Wort fermentability plays just as important a part in attenuation as yeast does. For example, different mash temps can change the sugar content of any given wort if you’re all grain, and different extracts have different fermentability levels depending on manufacturer.
By manipulating your wort you can use a less attenuative yeast and ferment the snot out of beer just by adjusting the mash temp or using enzymes to break up the unfermentable sugars.
Now, as boo boo pointed out, you not only have to take into account the fermentability of your wort you have to pitch the right about of healthy yeast to achieve the attenuation you want. It can be a very overwhelming concept to grasp, I’ve been consistently brewing for 18 months but have actually been brewing on and off for 10 years, with an 8 year hiatus, and only do I now have a good understanding of attenuation.
Here’s a few resources that may help you
www.mrmalty.com - check out the Pitching Rate Calculator
How to Brew - By John Palmer - Yeast - John Palmer’s Online Chapter on Yeast
The Brewing Network.com - Brew Strong: Brew Strong: 08-18-08 Attenuation check out this pod cast as well. Excellent advise!