Glow says it's peak day but chance of pregnancy is 30% why so low?

Hi!
​I have been TTC for 3 months. Just started to use Glow, I like the app but am a little confused. It has said today is my peak day but chance of pregnancy is only 30% ? why so low? I am over weight but when I played with the stats it made no difference to the %. I am logging my opk results and bbt so I am not sure what I am doing wrong! Any advise would be great ? x