MAVLink overhead

I am just looking at the MAVLink protocol. On Mavlink website, they said MAVLink 2 has just 14 bytes of overhead.

If I understand overhead correctly, it means that the extra data needs to be transported along with the payload. If we are not using signing, only 12 extra bytes are added on top of the payload. Doesn’t it mean the overhead of the protocol is only 12 bytes? instead of 14 bytes.