Let's look at what Fr. Bernas, an eye-witness to the proceedings,
REALLY said:
“The intention is to protect life from its beginning, and the assumption is that human life begins at conception, that conception takes place at fertilization. There is however no attempt to pinpoint the exact moment when conception takes place. But while the provision does not assert with certainty when precisely human life begins, it reflects the view that, in dealing with the protection of life, it is necessary to take the safer approach.” (p.78 Bernas, J., The 1987 Constitution of the Republic of the Philippines, Manila: 1996 ed.)
As Bernas said, the desire was precisely NOT to pinpoint the exact moment of conception. The decision was to ASSUME that it began at fertilization since that was the safer approach.
What part of that can't you understand?