I was able to quickly reproduce the issue. After some research, here is my explanation:
- The landing is a separated part, having 6 "faces".
- The flights will not include any part of the landing
Based on the 2 rules above:
- The first flight has 15 risers, the second flight has 4 risers. The landing has 1 riser
- The first flight has 15 treads, the second flight has 4 treads. Altogether, the 2 flights have 19 treads.
But based on local regulation, there might be different ways to categorize them.
I'm interested in your opinion about this!