Have you ever stopped to wonder why one person might be left-handed and another right-handed? Or maybe, why one person might be near-sighted and another far-sighted? Obviously, these are traits of the person's DNA, but what makes this happen?
Studies have actually indicated that the time of the year that you're born may actually have an impact on the genetic expression of your genes and affect the traits you have all throughout life.
As pointed out by The Royal Institution, this isn't a definite or conclusive answer to why these things happen to each of us, but it could contribute to them based on the way of life of the mother during the time of pregnancy.
For example, summer months lead to more sunlight, which in turn leads to more Vitamin D, as well as solar radiation, which might impact how you grow in the womb before you're actually born.
Obviously, not all mothers spend time basking in the sun like some others do, so this is where variation in this theory might occur.
Either way, it's very interesting to think about.