Not neccesarily down to the level. But more to each teams aspirations. No matter how poor Juve have been lately, their goal at the start of each season is to win things. Therefore Juve have higher standards when looking to buy a player.
You mean as in they are different types of players?
Yes, Ledesma is closer to Zanetti in that respect, but he isn't a significant upgrade over Zanetti(they're on the same level imo), the only Viola Central midfielder that Ledesma is considerably better than is Donadel.