I've seen it both ways.

Depends on the paint. Higher quality paints will get darker as they dry and lower quality paints (like flat) will get lighter as they dry.

from experience, i have seen both. as a diy homeowner, i must say it took a while to get used to that. as paintguy22 stated, it can be either or. one thing i found was stirring was vital. if my paint was not stirred sufficiently i got a different highlight or lowlight. just my 2 cents ~ :)