That may be an old residual thing from back before Debian switched to the same model Red Hat used. It’s been a long time, but that used to be the case. Red Hat was the first to adopt the convention of creating a new group for each user. Debian didn’t switch until a few years later, but it’s been a couple decades since they did.
You can configure it to do the expected thing, and I guess we need to update the defaults.
