I think the difference is that the two systems weren’t simultaneously used on the same vessel – (or at least I sencerely hope not) – my interpretation is that this is a migration from one system to another across the whole industry. A bit like changing from driving on the left to driving on the right (which I’m sure some nations have done). Might make sense for all sorts of reasons, but not without its risks. I doubt it was the engineers who were making these decisions too.

Cheers,
Tony