It takes great skill to match paint, especially if metallic. Rmember each mix requires x quantity of this and y quantity of that, what happens if a little bit of x or y gets stuck in the rim of the tin, or proprotionally more x than y gets stuck or drips? Yes you get a very very slight variation. Not to mention the paint being matched has had a few years of sunlight, causing it to change, plus then theres the actual spraying technique. This is no issue with flat colours, but on metallics is critical to getting a match. Unless the pain is sprayed in the exact same manner as the original then the 'grain' of the paint (the mica flecks) will not match the same pattern for density or direction and will make the paint look different, regardless of whether the colour is spot on.
On a dull day in the rain you'd never know my car is 3 different shades, but in certain angles of sun it's glaringly obvious.