You are absolutely right Leigh. Musn't mix up my transmittence and D values. D-uh.
Anyway, the general argument is that 0 D on a non-standard transmissive densitometer is actually 0 OD, and that once calibrated it becomes 0 ROD. There is no absolute scale as it is still calibrated only with reference to that specific instrument. What we want is for that instrument to give us sensitive and repeatable values, even if those values differ from some other instrument - and they will.