Yeah, if it's a different batch of the "same" colour, there's almost guaranteed to be some colour shift even if the formula wasn't formally changed between batches. Add to this that paint companies do sometimes tweak their formulas over time and you can almost guarantee that paint from different batches will be at least a little different. If the older paint ahs been in the sun or under strong fluorescent lights, then the old colour may have shifter even if the bottle colour didn't.
But, if it's the same bottle, painted soon after the first coat, then you may be seeing the effects of a more opaque layer of the newer paint on top of the older stuff which may be showing some of the underlying colour through.
Not a lot of help in any case, of course.