Because you walked into a building and entered the first floor. The apples are awarded to the floors visited, not the stairs climbed.
What have the stairs got to do with anything? They are irrelevant because there are many ways to enter a building at different levels regardless of stairs or elevators. As I said, any building built on a hill with have a different number of floors on different parts of the building so it is perfectly possible to walk from ground level into the 3rd floor for example.
Where my GF lived in Switzerland I would walk from the street level at the back of the building (with parked cars like any normal street) and enter the building at the 6th floor without taking a step up or down. I would then descent 2 flights of stairs to arrive at my GF apartment. This I would have visited 3 floors (6th,5th and 4th). We could then descent the otherwise of the building to floor 1 and get out at a new street with parked cars.
Stairs and ground level are completely irrelevant to the number of floors in the building and the floors that you would visit on a path from one location to another. What matters is the number of physical floors