Celebrating his birthday alone wasn't new for Zhang Jiren. In fact, this wasn't the first time he was spending his birthday on his own. He had no one to celebrate it with and he'd been long accustomed to the fact that he would always be on his own.
From the moment he was aware of his surroundings, he knew that he was quite different from his peers at the orphanage. Not because he was smarter than them, but because he felt that he didn't belong there.
From what he heard from the people who ran the orphanage, he was left there by an unknown man when he was around the age of two or three years old. Zhang Jiren didn't know about his real birthday until he stumbled and discovered the truth about his origin.