Chance of Pregnancy Percent Increase

Can anyone explain why my chance of pregnancy pregnancy percentage increased?
 I'm not on a birth control but my boyfriend and I always use a condom. We had sex on my first day of my fertile window and I logged that on here. 
My percentage went from a 27% to a 33% so I'm just curious as to why it's increasing. It's just giving me some slight paranoia lol. Thanks in advance!