The charts were drawn up by and primarily for the use of her majestys navy. Therefore it would stand to reason that some military/nautical termnology would slip into the mix. 'Zulu time' as Squig said, is the term used for GMT by the military and commercial sailing vessels the globe over. ...probably because 'zulu' doesn't phonetically logjam with the 100+ dialects in the world. can you just imagine a Chinese freighter captain trying to say 'Greenwich Mean Time'? Zulu is a simple 2 syllable utterance by comparison.
I guess it can be filed under 'just one of those things'.