When doing projections, if you remember that there are constraints in both team stats and league stats, you will prevent yourself from over-valuing players and your rankings and projections will be more accurate.
It doesn't matter if you are overprojecting or under projecting for players as long as you are consistent in your methodology from one player to the next.We're looking to relate one player to all othe players in rankings, whether or not you have 79 players projected for 2000 Rushing yards is irrelevant...until you start taking VBD into account, then you must make sure you are equally overprojecting for all other positions as well.
It does matter because not all STD DEVs are the same within positional ranks, and are definitely not the same when comparing positions.You do not evenly overproject for everyone. You (not you specifically, but the general you) overproject in a haphazard way that makes any VBD methodology you use worthless. I have my own thoughts about how worthless VBD is anyway but that's a different discussion.
That is the typical fallback of people who project stats for players without any regard to constraints. Your projections have to be constrained in a way that is consistent with league history. This means players on the same team have to be within team constraints. This means that players across the league cannot as a group all be higher than what players have been in years past.
There will only be so many 1,000 yard rushers in 2006. It is easy to justify why double the amount backs that typically rush for 1,000 yards could do it, but you'll be wrong overpaying for marginal backs letting value at other positions go.
This is the typical trap that people fall into with the RB "landgrab" that occurs in the first three rounds. People continually get burned on that next breakout player because they are hoping that RB X will hit the 1,000 yard rushing projection, even though they have already ranked more than the historical amount of 1,000 yard rushers already.
If you don't overproject you see the diminishing return of that marginal back in the 3rd round and all of a sudden there are some players at another position that stand out as a clearly better alternative.