Why can I do math quickly?

We had this "game" where each of us had to fill out a math sheet with 20 questions (division, addition, subtraction, multiplication). The twist is that if we "see" a division question, we had to mentally change the sign to another sign (addition) and do the same for other signs. I found it was easy for me to switch signs and mentally calculate numbers. Why is this? I hardly do math on a daily basis.