Qin Xiaoyou had absolutely no popularity when she first entered the school so Su Boyu naturally didn't notice her at that time. The reason he said that was to continue lying so as to cover up his biggest lie. Actually, he noticed Qin Xiaoyou because of Mo Wen. When he was investigating Mo Wen, he accidentally discovered that Qin Xiaoyou had close relationship with Mo Wen, even and ambiguous romantic relationship.
However, Qin Xiaoyou was the only new student who was on the List of Campus Beauties and was even ranked the third. His interest was piqued immediately as it was rare for a new student to have such great influence in Hua Xia University.
After that, he investigated Qin Xiaoyou's background and realized that she was just a normal girl-next-door. Her family basically did not have any background and was a single-parent family.