I suspect that some of their rankings derived from draft orders in actual drafts? I kind of figured that would be the case as they do seem to be shifting pretty constantly, which would be the case if all of the picks contributed to the draft order. This could also account for variances in rankings between leagues using different categories as the yahoo rankings. The definition says 'a player's ranking based on current and past seasons' which COULD also be interpreted to incorporate where they are going in drafts, as that is the only data that is available for the 'current' season.
From the perspective of the business, it seems like it would be a lot simpler to do that than to pay a team of 'experts' for 'expert opinions' (which might be where the rankings were on day one, and why they've 'shifted' a bit closer to 'reality'.... It doesn't really explicitly say anywhere that that is done but it kind of makes sense for an IT operation of that nature.