Heres the answer you're looking for I think. In 2012 for whatever reason, the mint ended sales on the 2012 sets early and moved on to the 2013. Instead of the usual time, the last month's of 2012 and then most of 2013 to sell the sets, they pretty much stopped right at the 2013 date change.
They aren't really low mintage compared to 2013 or 2014 sets either but here's the catch. When it ended early dealers got a bit crazy and spent more to buy them on the secondary market. This inflated the price.
Then in 2013-2014 the dealers and collectors speculated the mint would end sales early also, so they tried to get ahead of it and buy more sets when they were available, but they werent ended early, that only happened in 2012.
So to sum it up. It came down to public perception and supply and demand and dealer speculation. Truthfully there's enough sets to meet supply and demand for all 3 years, there shouldn't be a price difference but there is.
The difference here is because the mint ended 2012 sales early, dealers didn't get what they thought they needed for 2012 thinking they would be available like usual and ended up paying more for the sets on the secondary market inflating the price of those 2012 sets.
Then they depressed the price of the 2013-2014 sets by buying a bunch of them early.
2012 set prices are high because the dealers paid more to get then and now need to make their money back. 1. The supply likely exceeds demands for all 3 years. 2. The dealers paid more and have to ask more for the 2012 sets and that inflates the price.
2012 sets are overpriced and the only reason for it is because dealers paid more for the sets buying them on the secondary market for inventory.
I hope I explined it clearly enough, I'm not a great writer and I kind of ramble working out my thoughts. Anywho..