The reputation tab only lists rep changes within a range of dates, not the cumulative rep for those questions/answers. Apparently your upvote came on the day that you answered that question (
2010/12/25). I can see that on your rep page in the range
2010/12/26, you had 10 points rep from that answer. By default, the reputation tab only shows rep changes from within the last month which currently doesn't include that date. If you increase the range to include
2010/12/25 and the date your answer was accepted (
2011/02/27), you will see that it will be listed as having 25 rep gained.