There's no available setting for "Bunt for a hit". As far as I know, the Sac Bunt settings are unrelated to Hit Bunt attempts. If they are, that, in my opinion, is silly.
When determining if the batter is going to try to bunt for a hit, the engine probably considers speed somewhere close to first, but after the base/out situation. It probably adjusts for other offensive abilities. Wouldn't surprise me if RC/27 or OPS/OPS+ got factored in. Better = less likely to bunt for any reason. Crackpot theory, I know, but it'd make sense in a quick and dirty kind of way.