The fellow above has posted good reasons. Yet there is another, which is related to what you mention and it's possibly correct. CX is known to, sometimes (but certainly not always) upgrade members who are bumped up a tier the first time. I've seen two colleagues get upgraded their first flight as SL. Personally, 12+ years ago when I first hit SL just starting out with CX I was given an op-ups HKG-SIN. And I've read about it on here.
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Now I've never heard about this dynamic before for Green, but I think it is at least a possibility you got the upgrade as your first flight as an MPC member.