Do some research, many brands don't use pure or high quality olive oil. Some have been proven to actually be presses of the leftovers from other producers that are used and old. Sometimes pressed pits which give a sour taste, or cut it with cheaper oil.
You wanna be sure it is 100% olive oil, and preferably first press.
In short: The label means nothing, you actually need to do a small amount of legwork. But once you know the brands it's easy.