Birthdate v. Due Date

I'm going to post this here because it got swallowed up by all of Mr. Nice's threads under the Astrology board:

I apologize if this has been asked before, but I can't find it and I'm not entirelyl familiar with decans, ascending, moon, sextile, squares, etc:

I was born April 30th, but my due date according to my mother was May 25th, making me about 3 weeks early. How does the science of astrology work? Even though I gestationally was to be born on May 25th, how is it that we develop the traits of the sign in which we actually were into? Had I been born on my actual due date, that would have made me a Gemini with a Cancer moon, resulting in different charateristics, right?

I don't deny I'm Taurus because I have MANY of the traits, but I've often wondered this.