I had some atlas grey mixed up in aerosols and the paint supplier used a tool which scanned the paint then shaded it. Looked spot on until I put lacquer on then it was a shade too dark.
A heavy or light application can vary the shade out of same tin. a few heavy wet coats can lead to a darker shade, while light dry coats can give can "Float" the aluminium flecks and make them more visible and reflecting more light, making the shade appear "Lighter"
Lacquer applied too heavily can also change the shade, usually darker. most will try and achieve a glossy finish with an aerosol, and put it on heavy.... Light even coats with 5 to 7 mins between coats is the way to get a colour match, the gloss will easily come with a bit of elbow grease
Thom