Can someone with extensive football knowledge explain to me how Mariota is a projected top 5 draft pick, while some say Miller may be a 3rd or 4th round draft pick?

If Miller continues to improve his passing and plays the way he played in the 1st half of the Wisconsin game, while staying healthy all year then I believe he will be a 1st round draft pick.

https://www.youtube.com/watch?v=q4mLEuvE8AQ