Usually those dozen ingredients are things like spices which you might only buy once every 6 months+ (so next time you make that recipe there are only 4 ingredients) or they're staples (flour, pasta) or fresh consumables which are shared between many recipes (onion, garlic, cheese). So if you're cooking at home a lot your grocery list is often the same thing every time plus a few odd items periodically even if you make different things every week.