The twins I have known are the same. I would assume it has something to do with a desire to differentiate themselves from one another, but they always seemed far more dissimilar in personality and affect than my siblings.

OTOH could be siblings tend to be more similar as the smaller ones try to copy the older ones (my son would dress up as a ballerina to play with his sister when he was little, my smaller brother would acquiesce to play chess with me just to spend time together etc).

I have given up trying to explain child development, there's just too many variables.