Who or what is attributing their coins?

I purchased what I thought was a 1823 Broken 3 Capped Bust Half Dollar from Heritage Auction on 3/19/11. The PCGS holder designated this variety on the holder. I listed this coin on ebay. I received two emails through ebay that this coin was not a broken 3 variety. I looked at the coin more closely, and determined it was not this variety. I canceled my auction. I contacted PCGS through email, I was asked to submit a photo of this coin. Several days later I was asked to send coin back and label my shipping label with "Mechanical Error". I received this coin back with no attribution, just a plain holder. No explanation, nothing to compensate my lost. Does "mechanical Error" mean that a machine graded this coin?, so PCGS is not responsible?.

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