Factors Influencing Attentuation
The apparent attenuation range published by yeast manufacturers shows how the yeast generally perform, under typical fermentation conditions. The following factors will affect the attenuation delivered by the yeast:
Fermentability of the wort
Maltotriose is only partially fermentable, and dextrins, caramelized sugars, and some adjuncts, like lactose are unfermentable, so having a higher concentration of these will result in lower attenuation.
Also, keep in mind that adding highly fermentable adjuncts will increase the fermentability. Late additions of highly-fermentable sugars can drive attenuation well past the typical range.
Lower fermentation temperatures result in slower yeast activity, which tends to cause under-attenuated beer. Higher temperatures (within limits) increase yeast activity, helping to lower final gravity.
Lack of oxygen early in the fermentation stresses the yeast and limits growth, causing lower attenuation.
Pitching the recommended amount of yeast will produce the healthiest colony, which will in turn achieve higher attenuation. Underpitching stresses the yeast, limiting growth, and overpitching creates an environment where yeast don't have enough nutrients to grow healthy.
Providing proper nutrition supports healthy yeast. In contrast, lack of nutrients, specifically zinc, can lead to stalled fermentation.
Rousing the Yeast
This is a bigger factor with more flocculent strains, where healthy yeast will drop out of suspension early. Rousing the yeast also drives some excess CO2 from solution, helping the yeast to lower the final gravity.
If your apparent attenuation is drastically higher than expected, the beer may have been contaminated.
Errors in Gravity Readings
While not actually a factor of attenuation, it's important to remember, as Brewchez mentioned, that there is some error inherent to your measurements.