I assume the seat numbering is done according to fictious compartments, 1 beeing the First 2 rows of seats closest to the door, in first class this would be seats 1 to 6, combined with the compartment number giving 11 to 16, the next 2 rows would make 21 to 26
most coaches have 7 fictional compartments:
entrance (11 -18)(21-28)(31-38)(41-48)(51-58)(61-68)(71-78) end
yet the end coaches next to the power units have the numbers more mixed up it seams
I think we can conclude that the numbering is a heritage of the formerly commonly used compartment coaches on long-distance trains