You'd actually be surprised how different the color can be on two cars of the same model year in the same color. No two batches of paint are the same, just try and paint half a wall in your house with a new bucket of the same color, it will be VERY noticable.
Thats why they have to blend. The difference is less noticable when its spread out across a panel then if its clearly one shade on one side of a panel line and one shade on the other.
Now, there are shops that have new computerized paint analysis machines that tint new paint so that it exactly matches the old paint, including the differences in factory color and differences due to age. They don't blend, but they are 150-200% more than the competition that does blend. They charge $600 just to repaint a bumper for instance... Those machines cost $30-$100,000 depending on how complex they are.
BUT they do unbelievable work...
So, ask around. I know after seeing the work of these guys with the paint matching machine and the work of people that blend that I would never be happy with paint that was blended. But, had I never seen their work I may have been.
One big thing is the flake. When cars are painted at the factory the metal is electified with a current to make sure the flake all lays the same way. Shops do this also, but its harder to do when blending then when repainting just one whole panel.