Is there any significance to the 6 doors the Master Key opens?

I was baffled to learn it only opens six doors throughout the entire game. It feels *far* more useful than that. But this has me wondering- is this key purely a feature of gameplay, or is it a diagetic item that implies these doors may have been built or secured by the same locksmith? Or for some related purpose? And what of the exceptions?

The six keys it replaces are:

Residence Key (Undead Burg)

Watchtower Basement Key (Undead Burg)

Mystery Key (Undead Parish)

Sewer Chamber Key (Depths)

Key to New Londo Ruins (New Londo Ruins)

Cage Key (Sen's Fortress)

In terms of "exceptions", the first and last keys on that list are still required for *one* door each, with the Master Key being unable to unlock Griggs or Logan.

Even if there's no established explanation, I'd love to hear theories :)