Got pregnant from the pull out method

So my husband and I have been doing the pull out method for 3 years (birth control really messes with me and my hormones) and I haven't gotten pregnant. Well, I was late for my period and took a test and it was faintly positive. The next day it was darker. Then later in the day I started bleeding and it went to a full blown period so I am assuming this was a chemical pregnancy. We decided after our second that we would be done having kids. But after this experience I cant help but wonder if I really am done because I have been pretty sad the last few days. Anyway, my question was, who has experienced getting pregnant from the pull out method? We are very careful, avoid ovulation days, and his pull out game is strong lol. It just makes me wonder how it happened after all these years of it "not" happening.