As far as I know, the achievments stats page only seems to count most stuff from the day that the achievment system was introduced whereas the honor-kills on the main profile page would have been counting from day 1 of the character being createdOriginally Posted by telemain
For example, I cleared almost all of the Vanilla & TBC raids way before the date shown on the achievment system but they only show as being done on the day the achievment system was introduced.