How is glows chance of pregnancy calculated?

How is your chance of pregnancy calculated? during the fertile week each day you are given up to 33% chance of pregnancy. Other days are 3%. On 3% days, is your chance of pregnancy because the sperm could hang out and survive until ovulation occurs? During fertile week, why doesn't it get higher than 33%? Is it because an egg might not actually be released, the sperm might not be able to penetrate the egg, etc? Just curious how it's all calculated.