Tigger, while I too use Wikipedia I *always* take what it says as a guide for more research, not as gospel.
In years gone by Y meant economy class. Then the airlines stated yield management and the system described by Capteonian spread. However, in the minds on many people the idea that Y = economy was (and is) still prevalant. To try and not confuse people (!) some people in airlines and agencies seem to think that if they use codes like Y/M and Y/B people will understand they are booked in economy at the M or B fare level.
And, to go slightly off topic, even airline people don't understand this fare class lark. Some years ago I had a Star Alliance Business Class RTW ticket. I therefore had a ticket where the fare basis for the whole ticket started with C (can't remember all the other letters). But I had been booked on a flight which only had economy class from Kuala Lumpur to Bangkok so the class of travel for that segment was shown as Y. I turned up at KL airport to fly to Bangkok in time for an earlier flight that had Business class and which had space in both Business and Economy available. The staff point plank refused to put me in Business because they said the class of travel for my ticket was Y - they failed to realise the difference between booked and paid fares.