It amazes me that fans think teams draft pure BPA.
Coaches suffer through the entire season with players they would love to upgrade, and when the draft finally gets there and they're not going to try to upgrade them? Won't happen!
Part of the BPA legend comes from coaches not wanting to damage a player's confidence. Coaches don't say "We drafted John Doe because we needed to upgrade our Left Guard", because that could be devasting to the LGs confidence and his trade value. Instead, the coach says "We couldn't believe John Doe was still available, he was the BPA so that's why we drafted him".
Do coaches draft BPA, even though the position is not weak? Certainly sometimes, but I bet upgrading weak positions is the first priority and BPA is second.