Well, my answer has to be a question- where did the figure for the estimated Fg come from? If you estimated the FG based on the yeast strain's probably attenuation, your ingredients, and your mash temperature, then it's possible that you made a mistake in estimating the probable FG.
If it came from Beersmith or other software, then it won't be accurate. Beersmith just gives you a 75% attenuation guestimate, no matter what the yeast strain or ingredients or mash temp is.
In my last beer, I also got a 1.008 FG. But that was because I mashed (on purpose!) at 150, and used a highly attenuative yeast strain.
Ingredients play a huge part in the FG, as does the mash temperature. If you used some unfermentables like lactose in the beer, or less fermentable ingredients, you'd have a higher FG. If you mashed at a higher temperature, then you'd have a higher FG.
If you did mash at a high temperature and still got 1.008, I'd double check the calibration of the thermometer.
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