It is indeed not a valid assumption that the positions listed on a player's editorial page will correspond to their eligibility in the fantasy game. That is part of where you are getting crossed up. Since fantasy bases positional eligibility on past performance where possible, you might see a player eligible at a position that they really don't play anymore. To take an example from last season, BJ Upton had infield eligibility all season, but if my memory serves his fate as an outfielder was pretty much sealed from the beginning of the year. If he played enough infield to qualify last year, it just means my memory is bad, not that the explanation in general is wrong
The editorial (sports.yahoo.com) pages on the other hand, are just that. More like news. If someone goes there to learn about BJ Upton, it would have been a bit odd to tell people he was an infielder when he really wasn't and hadn't been for some time. Fantasy uses different rules since we're trying to make a fair, predictable game based on a real-life league that doesn't have the courtesy to manage their players with our game in mind
The sticky part comes when a new player enters the game. I can't say much yet about what happens this year, but I can at least catch people up on how things worked last year. Last year, when we saw a player had entered the game, we would base their positional eligibility purely on observed data: where they played that year, and if they hadn't played that year, then we'd check the previous year. The place this caused a lot of tension was with rookies, where there was no data to observe. The way the game behaved last year in this case was that it would wait to see where they first officially played, and would grant them that eligibility. In a few select cases, based on overwhelming public demand, we forced a player into the game prior to that, and subjectively chose a position for the player. An example would be Max Scherzer who we forced into the game as a relief pitcher based on published reports that he was going to be used as a relief pitcher even though he is likely destined to be a starter eventually.
That is how it worked last year. As I mentioned in another thread, I can say that we've made some improvements to how we manage our player pool this year, but I can't yet detail what those are.