Caculation of the crop in the field is not like its calculation at the threshing floor.

the meaning of the proverb is as follows:
the amount of the wheat when collected from the field does not remain the same as the amount being processed at the threshing floor.

You may also say:

counting your crop at the field is not like the counting of it at the threshing floor.



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Note added at 2003-08-03 09:31:49 (GMT)
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calculating the crop in the field is not like that at the threshing floor.

This denotes that while the crops still unharvested they weigh more than have been harvested due to the loss caused by different factor, i.e., there is a difference between the theory and realities on the ground. The English proverb also could apply 'do not count your eggs before they hatch'.

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Note added at 2003-08-05 19:03:17 (GMT)
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Distance between theory and practice is like the distance between dreams and realities.

My play was a complete success. The audience was a failure.

...

This is the meaning implied by the proverb: "When ends do not meet"