When playing as Slavs, other civs Siege weapons also look cheaper when viewed from the tech tree. Makes me wonder how much “bleeding effect” there is across tech trees.
Tech tree always displays units stats that your own civ currently has. This has been a thing since AoK.
This is a bug that needs to be fixed then.
Thank you for the report! This issue has been written up for investigation by the team to see whether improvements can be made!