Wanted to address your points: As you explained, fluke means accidental or lucky. The word suggests you're giving credit to outside forces. Calling something a fluke in hockey is saying a team wouldn't have scored a goal or won a game without something helping them along, like a puck bouncing off the ref's ass into the net or a weird bounce off the stanchion.
Montreal's playoff run was totally unexpected, but what lucky or fluky thing helped them? I honestly can't think of a single goal or win or individual performance that was a fluke. No Fernando Pisani coming out of nowhere, no stanchions or weird bounces. Montreal's best players had a ton of injuries during the season, then they returned for the playoffs and played like their best players. I don't see any luck/fluke involved (if anything, they had BAD luck during the season). People mentioned rosters from different years – what do different rosters in different seasons have to do with that year's roster?
For me, the misuse of "Fluke" is like people misusing "They're", "There", "Their". I don't care if someone casually mixes up "Fluke" and "Unexpected". However, a couple of people in this thread seem oddly motivated to keep misusing the word. Whatever their agenda, they're not interested in analysis.
EDIT: Last night's date was amazing. This brilliant and attractive woman was attracted to me. Now THERE'S a real fluke!