could be volume...make sure for a five gallon batch you are EXACTLY at five gallons.
for most extract brewers the problem is that the sugars settle so you don't get a good reading. you need to mix it thoroughly just before you take the sample so you don't get a diluted sample or a sample with a heavy mix of sugars.
if you ARE doing extract, i would just go with the projected gravity and trust it...it will have exactly how much sugar you put into it.
EDIT: Final gravity can depend entirely on yeast attenuation, ingredients, mashing temperatures and a number of other things. you generally did not get anywhere near my projected FG when making extract batches. as long as it doesn't taste too sweet, you're in your attenutation range (usually ~70%) and there is no activity for several days, you are ready for the next stage.