It's not undervalued, it's supply and demand. There are a ton of them out there in high grades. The 1909-S VDB is the same way. Because it was a first year issue many were saved and preserved in high grades. Sure, in the lower grades it is expensive...but it doesn't increase in value at the same rate as a 1914-D even though 3x were minted. In MS grades, the 1914-D is much more valuable because there aren't nearly as many left. Same is true for the 1931-S. Sure, it has a low mintage...but a large percentage of that low mintage survived and as a result it's not overly valuable. There are similar examples in nearly every series.
If you read this entire thread, I think you may find that the 31-S is overvalues and the 14-D is undervalued. Just as a for-instance, there are only 297 NGC and PCGS certified red 1914-D's. However, there are 2123 1931-S's certified by PCGS alone and there is a much lower percentage of the 31-S's certified because they are worth so much less. This thread accounts for nearly 1/4 million 31-S's. That is nearly 25 times as many coins as there are 1914-D's know. BTW, since you are in a correcting mood, "serverly" is severely and your other misspelled word probably should not even be used in this forum.