I wondered how much it changes every year, if there is a pattern to it (the amount and direction of change) and what influences it. As a former astronomy student, I should know this -- but I don't remember the topic being raised (other than the general dates of the solstices and equinoxes). Hmm, maybe I still have a text around here somewhere...
Ahh, the internet saves the day -- of course -- the differences are due to the externally imposed requirements of the calendar and human measurement of time.