ᐅ Older house from the 1970s – were the doors made smaller than usual?

Hello,
during a viewing of a property, I noticed something that even the real estate agent couldn’t answer.
The property was built in 1978. Apparently, in the mid-1980s, the then-owner made the door opening between the bedroom and the children’s room smaller—so narrower. Unfortunately, I didn’t take a photo. But it was originally a standard door frame, and then a plastic door frame was added on top. Approximately 3cm (1¼ inches) on each side.

Question for the experts: Why would this be done? Was a new door installed? Is this common for all doors?

Thanks in advance if anyone knows.

This answer sounds awkward, but honestly: how are we supposed to know? Ask the owner, pester the realtor, check it out on site, or post some photos here. Maybe they needed space for built-in wardrobes, or space without an obstructing door leaf????!

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