At a guess, it's just that it takes much longer to shift stock at that price point. More people buy the lower-end chips, as the ol' bang-for-buck ratio goes fairly crap after the 3500+.
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A64 3200+ Winchester @ 250x10= ~2.5Ghz, ~1.41 Vcore
1Gb @ 209Mhz, 2T, 3-5-5-10
Voltmodded (1.99V) Sapphire 9800Pro @ 460/350 w/ modded VGA silencer 3.