I used to think it was to do with the quality of eye sight but was really fascinated to learn it's more about what your brain chooses to filter out. Neither version is better or worse thankfully!
For those who've only seen it one way, these are the two versions most people see and what the dress is actually like.
Is the first picture the one that was originally there. I could have sworn I saw a white and gold dress the first time I saw it but now I can see a black and blue dress.