Because there are no sources!? You’re just making shit up. The reason the 140mm ASCALON has a relatively low pressure is because of a large chamber volume. Such a large chamber volume is physically impossible with the 120mm ASCALON.
You mean if increasing the size reduces the amount of pressure? You really want me to google the definition of “pressure” for you?
ASCALON is meant to fire cased telescope ammunition which inherently has a greater amount of propellant. Bigger explosion + smaller chamber (since its a 120mm cannon) means its impossible for the 120mm ASCALON to have the same amount of pressure as the 140mm. Or do physics not apply in your Wehraboo wet dream where only Rheinmetall can make effective cannons?