Same experience here. Some of my skeins end in 05, some in 05 06 and some in 06! I've knit them into a blanket with no variations in color.
I assume there is more "science" than "art" involved with acrylic yarns. It's not like the hand dyed yarns, dyed in small batches, that absorb at different rates or have a more human factor involved.
Maybe the color variations were the result of lighting or a ball that was wound more tightly/loosely or in a different direction.