Even if working out of the original can, over time the paint on the
wall will begin to oxidize and darken a bit, making an exact match
impossible. Unless you know the paint brand, line within brand, and
color you're unlikely to even get close. Add to that the fact that the
manufacturers have had to reformulate products to conform to VOC
regulations and thus even the same color is no longer exactly the
The answer to your question is yes, you are expecting too much.