It's because, bless them, they think "perfect" always meant what it does now.
In 1605 (?)(or when the King James Bible saw the light of day), the meaning was more like entire or whole.
cf Macbeth Act 3 scene 4
MACBETH
Then comes my fit again: I had else been perfect,
Whole as the marble, founded as the rock,
As broad and general as the casing air:
But now I am cabin'd, c
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