For one, thanks for the pic, for two, thanks for carefully measuring it and for three, WOW!!!
As for a 2 or 3 inch discrepancy, I suspect that unless it's anesthetized or dead you'll never get a truly reliable measurement even if the snake cooperates, and a 2-3% (as is the case in your very minor miscalculation) tolerance range could be expected. This is because the snake will always have some capacity to either traction or compress its vertebral column. - No science here, just my guess.
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