Old building from the 70s - Were the doors made extra small?

Hello, during a viewing of a property I discovered something that even the realtor cannot answer. Property. Year of construction 78. Apparently, in the mid-80s, the then owner made the door passage to the bedroom and children's room smaller... that is narrower. Unfortunately, I did not take a photo. But it was the normal door frame and then a plastic door frame was added. Easily about 3 cm on each side.

Question to the experts. Why? Was a new door installed there? Is that the case for all?

Thanks if someone knows.

Reads stupid, this answer, but honestly: how and why should we know that? Ask the owner, pester the realtor, analyze on site or post photos here. Maybe they needed space for built-in wardrobes, needed space without a disturbing door leaf????!

Basically, it doesn't matter at all. If you want wider doors in the current standard size, just widen them again. It's not a huge effort.