This happens every year where we see players being reached for and others falling for inexplicable reasons - see Clellin Ferrill (taken at #4) for Raiders last season and Justin Reid (falling to the 3rd) a few seasons ago for us.I think all of these QBs will be there when the Texans pick. I've seen Grant going from the early 2nd as the first Safety off the board, to late 3rd as the 6th Safety taken. Grant's draft slot really depends on how much teams value production (good for Grant) vs upside (poor for a player who will be 24 when he plays his first game).
Different teams assess and value players according to their schemes more often than not - even while they claim they draft according to BPA.
BPA is an ambiguous term because the best player available would be the one of most value to your team and the first round picks are valuable and must hits.
This is where the best national mockers have their work cut out for them - they have to know the various schemes on D and O of every team and then match them with the skillsets of the draftees.