Why does WOODEN-DOOR have TRANSBIT?

[eriktorbjorn]

<OBJECT WOODEN-DOOR
        (IN LOCAL-GLOBALS)
        (SYNONYM DOOR LETTERING WRITING)
        (ADJECTIVE WOODEN GOTHIC STRANGE WEST)
        (DESC "wooden door")
        (FLAGS READBIT DOORBIT NDESCBIT TRANSBIT)
        (ACTION FRONT-DOOR-FCN)
        (TEXT
"The engravings translate to \"This space intentionally left blank.\"")>

From what I understand, the wooden door shouldn't need TRANSBIT unless it contains other objects. Which I don't think it does. This is how I've seen it documented:

"TRANSBIT: The object is transparent; objects inside it can be seen even if it is closed."

"12.1.7 TRANSBIT
Indicates that the OBJECT (a container) is transparent. The fellow who prints the description of
OBJECTs within ROOMs won’t tell the player what’s in a container unless it’s open or transparent."

The only objects in Zork I that have TRANSBIT are RAISED-BASKET, TROPHY-CASE, BOTTLE, WOODEN-DOOR and GLOBAL-OBJECTS. (Though it seems the parser can set the bit, at least temporarily, in some cases.)