Only 50% bonus damage against cities for siege units? (3.15.1, newest)

[Enrico Swagolo]

Is it supposed to be like this or is it a bug? In my Japan game my Samurai were much more effective against cities than my Trebuchets which was very weird. The Samurai can easily get a 75% bonus with one promotion, while Siege Units seem stuck with only a 50% bonus, unable to get it up.

My Trebuchet did only small damage to cities while the Samurai were obliterating them much more effectively. It seems like a weird design choice - if it's not a bug, that is.