Have you leveled your table with a real level?
Alright I did some leveling with my dad's leveler and he also helped me check if I've got the same result on my iPhone and mine is actually pretty darn accurate, so for me mine is really spot on!
He also said that the iPhone isn't flat so you can't get an 100% accurate reading.
I have other issuess with the accelerometer, like when I'm in bed the accelerometer can freeze in one mode, anyone else expering this?