Gift bits when encoding a sequence of messages, how is that?
The important thing is what "information quantity is additive" with respect to.

Information quantity is additive with respect to concatenating messages

each of which can only have one length, rather than forming a multiset

of messages each of which can only have one length.

Information quantity is subadditive with respect to forming a multiset of messages

each of which can only have one length, because for every concatenation,

there is a unique multiset that is consistent with that concatenation.