2008 proof set silver cheaper than clad?

Why am I finding the 2008 silver mint set selling for $15-$20 cheaper than the clad set. This makes no sense to me. Has anyone got an answer?

 

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They cut off the sale of the clad ones very early and very suddenly. The mintage was low and a lot of collectors must have been putting off buying them and want them now.

 

The 2008 Silver Proof set is still available from the Mint, which seems unusual to me.