According to Edward Teller as quoted by John McCarthy, the miscalculation may have been intentional.
See
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-Andrew
jtaylor wrote:
> The Germans were pretty much on their way to having their own atomic >> weapons.
> The stuff I've read (Farm Hall transcripts, for instance) says no,
> they were working on a pile, not bombs. They miscalculated the amount
> of fissionable material necessary and so thought they could not
> a) get enough;
> b) if not a), get such a big bomb to anywhere it would do them any good.