Hello Craiden
Well spotted!
It has historical reasons and comes from the times when the aisle in the waggon was not in the center but at the side and the waggon consisted of closed 6 and 8 seats compartments. However, it is not a Swiss invention, as many European coaches use the same numbering pattern. It has the advantage that coaches can be exchanged without messing up the seat reservations.
Furthermore, there is no seat 1, as the numbering always starts at 11, and this in the first compartment on the left hand side in the direction of travel. Number 11 is the window seat. Opposite it is seat number 12, while the seats next to it - i.e. those in the aisle - are numbered 17 and 18.
The aisle has been relocated: In old days, the aisle was not in the centre of the carriage, but at the side.
At that time, there were compartments of six and eight in the trains, and the seats were numbered in sequence. The first compartment therefore contained seats 11 to 16, but when the aisle was moved to the centre of the carriage, two four-seater compartments were created on the left and right. Seats 13 and 14, which were in the centre, were removed. They are still nowhere to be found on the train today.
Kind regards,
Roland