Why are olives always better at the restaurant?
Assuming that you are buying from supermarket shelves,(and not, say, from olive bars that some supermarkets have) there is a quality difference between shelf-stable canned and jarred olives vs. olives available in olive bars like Whole foods or in a Mediterranean deli. These come oil cured or submerged in water, and cannot be stored for long term, and they taste much better.