...I think the explanation is probably pretty simple.
Something like this : On the Gold RM011s, titanium has always been used (apart from a few LEs like the 'Ivory', etc) for the case centre to keep the weight down, and as the case back was marked RG or WG, with corresponding hallmark stamps, the centre was marked TITANIUM to avoid any confusion and to ensure the buyer was aware. Maybe even by law ?? This marking wasn't necessary of course on the Ti versions and was definitely absent (I have had the good fortune to handle quite a few)... Moving along to recent production and today, I am guessing in an effort to simplify and streamline production and assembly (maybe especially with the new RM case manufacturing facility ) it just made sense to standardise the process. And TITANIUM looks cool on the case anyway. IMHO

That's just my two centimes