I am about to ovulate so why?

Ok so iam usually very good with pinpointing ovulation and all that. I for the fun of it went to a site where you can use the ovulation calender. I punched in my last period date which was Jan 10 and am on a 28 day cycle. So i would ume that I would ovulate around Jan 24. The calender said i would ovulate on the 22. To me that dose not sound right. Has any one ever used the ovulation calender's and it give you a totally different answer from what you thought? Any ways I think I am to ovulate on Jan 24 so I probably should just start trying tonight, tomarrow and Sunday which I would think is the correct date. Then again maybe Monday too.