A couple possibilities:
-Are you reading the hydrometer correctly. Most you're supposed to read through the meniscus, but as far as I'm aware, there are some out there that you're supposed to read above. That could make you a couple points high
-Are you properly correcting for temperature?
-If you're brewing extract batches, they should not be off. If you're adding the right fermentables in the right amounts, and have the right batch size, they will not be off. Different amounts of fermentables, different kinds of fermentables, and different batch volumes will change the gravity. But for a known amount and combination of extracts/sugars in a known volume, the gravity will not be wrong, period. If you're topping up with water prior to taking the reading, you're probably not mixing thorougly enough.
-Are the volume markers you're using accurate? If your going off the lines on a bucket, measure yourself. If it says 5 gallons and it's really 4.75 gallons, that'll explain it.
-Some extracts have a habit of being slightly more unfermentable than they should be, and finishing a little higher than intended. Doesn't mean a stuck fermentation, but rather that the yeast have gone through any available fermentable sugars, and there's a higher than intended amount of unfermentable sugars.
Up Next: Lamebic, Flanders Red, Malt Liquor, Belgian IPA
Primary: English IPA, Spiced Winter Warmer, Brett. C. Old Ale
Secondary: Sour Stout, Brett C. Wild Bitter
Polypinned: Best Bitter
Bottled: Sticke Altbier, Doppelsticke Altbier, Weizenbock, Berliner Weisse, Spruce Brown Ale, Arrogant Bastard Clone, English Summer Ale, Coniston Bluebird Bitter Clone v1.5
In the "cellar": Brett B. Tripel, Quadrupel, Tripel, Brett C. Oaked English Barleywine, Lamebic