White doesn't equal white!

I am currently painting my new place.... well, I ran out of white. There was a patch about about 2 feet by 2 feet at the corner of two walls that I didn't have paint for. Shucks. So I headed out and bought new white paint, the exact same brand that I had before because yeah, white doesn't equal white - and wtf, even the same brand white was a tad darker! True, the one started with was a leftover from painting my parent's living room last year, but you'd think it would still be the same... noooooo. So I waited until the paint had dried to see if maybe it was only cause it was wet paint that is was darker. Nope. Dry paint is still darker. Grrrrrrr. So I ended up repainting two walls *ack* Which put me way behind in my painting plan *sigh* So I have to head back tonight to finish some tidying up painting before I can go back tomorrow to paint the ornaments I wanted to paint on the wall.