First, you need to define 'nerd'. I assume you mean someone who is socially clumsy but very clever in some respects. Also, what do you mean by 'different to a normal person'? In what way different?
A general image of a nerd is someone who is not physically attractive, perhaps unkempt, but a computer genius or very intelligent, who has not much interest in sex. If this is the stereotype who refer to, then there is an answer based on genetics and evolution, although it is not at all proven. It is merely a speculation and a discussion point for general curiosity.
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The reason is that, as nature generally tends to create more complexity and intelligence, people who are already very intelligent do not rely so much on procreation in order to have progeny who would eventually evolve into something higher. They don't need to, as they are at the top of their niche themselves, from nature's point of view. So, they do not need to attract a partner. The same can be said about weird or unkempt professors (see Einstein, Hawkins, long-haired, long-bearded eccentric intellectuals, etc). They don't need to be attractive from the point of view of evolution because they are already at the top and they don't need to improve further.
Continuing this line of thinking, and always for amusement, this explains why there are so many jokes about dumb blonde women. Because they are 'dumb' they HAVE to be attractive in order to attract a partner and thus hope that their offspring would be selected through evolution and eventually attain higher intelligence through this way.
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