Newer Proof Sets

They have more coins in them with the dollar coins now. Theyre also packaged better

I think you're asking about buying "old" newer proof sets rather than new from the mint, correct?

If that's the case, typically people collect proof sets by year when they come out. As icollectyoubuy said, it has to do with demand. As soon as its 2013, everyone wants a 2013 proof set. A few people never got around to picking up 2012, so they might look for one of those in 2013, too. And so on down the line. Depending on mintage, more common proof sets that are more than a couple of years old have much lower demand.