The measureables you see above are official as deemed by the manufacturer - American Apparel - which is useful if you're diligent enough to pull out a tape measure to compare against that favorite shirt of yours laying in the closet. You know the one.
Otherwise I'll say this, these shirts run smaller than your standard Hanes/Fruit of the Loom variety. Particularly the ladies shirts. Please heed the charts. Let's get it right the first time.
American Apparel shirts are form-fitting in style. This should be evident in the photos. Mens shirts are maybe a half size smaller than a Hanes Beefy Tee in my estimation. The ladies size chart recommends number sizes for each of the 4 shirt sizes offered - ie. size Medium for the ladies who wear size 4 to 6. This is a good indicator. Use it. Otherwise, if you're on the fence go for the larger size. This is my advise.