This is actually a great beginner question!
Yeast will consume whatever is fermentable, so as you increase your OG (add more stuff for it to eat), your FG will also increase slightly (the stuff you added for the yeast to eat also has a small amount off additional stuff it can't eat).
If you use a brewing calculator and punch in the numbers for your brew, you'll watch the OG climb. For example, for this recipe, with 5lbs of DME you end up with an OG of about 1.044 and finishes at 1.011 - but with 6lbs of DME, the OG climbs dramatically to 1.053 yet the final gravity reading only rises slightly 1.013 - because you added more stuff, and a little bit is stuff the yeast can't eat.
So, to answer your question simply, it can only "do a certain amount." A yeast won't always finish as the same gravity, like Danstar Windsor won't always finish at 1.012 - it's all about the stuff it can or can't eat in the beer.
Edit: Also, keep in mind that yeast is a living thing and not always predictable. There are occasionally deviations in gravity readings, but they should be small differences.