Yeah…so…you know when you purchase a house and the prior owners leave all the various left over paint in case you need it for a touch-up…and you go through it to find the correct color and you find one that looks real close…and then you are painting with it and it looks a bit too dark…
Flats can get lighter a tad as they dry depending on viewing angle.
Semi gloss etc go darker as the “wet” sheen fades which is glossier than the final result.
Trick we did at the hardware store:
Dip the stir stick in and wipe it on the rim of the can leaving an full thin even coat of paint on the stick. Dry it with a hair dryer on medium from a few inches away so that it’s not baking it, by just warming it. You can wave it across to keep it even.
That’ll give you a rough idea, if it appears that it didn’t cover well - dry a bit more and try a second layer. If it’s still off then it’s a bad mix / wrong colour / different finish.
Also your colour chips will rarely perfectly match as they are typically made with acrylics like nail polish and not paint - so it’s not technically a fair comparison.