why are silver proof eagles vary in price ie, 1992 $104 while 1991 is $31 is it simply scarcity?

Silver Eagle Proof Coins Ending Soon on eBay:
2008 SILVER PROOF EAGLE NGC PF69 ULTRA CAMEO-GRADED VIRTUALLY PERFECT GEM BEAUTY

US $84.95 (0 Bid) End Date: Tuesday May-22-2012 17:12:52 PDT Buy It Now for only: US $93.50 Bid now | Buy it now | Add to watch list

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  A LOT OF THREE PROOF SILVER AMERICAN EAGLE 1999 ~ 2001 COMPLETE WITH OGP AND COA $150.09 59m

  2012-W Proof American Silver Eagle $1 NGC PF70 PF-70 Ultra Cameo Early Releases $109.00 27d 22h 31m

  2006-P 20TH ANNIVERSARY REVERSE PROOF SILVER EAGLE NGC PF69-BLACK LABEL BEAUTY! $217.50 3h 34m

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