Mae misidentifies the murder weapon as a sickle. Considering her story about using a sickle regularly to clear weeds (which, incidentally, would not work) she should have known that the blade is too small to have made the wound on the victim. Sickles are single-hand tools used to cut and gather crops during harvesting. They're also usually only sharpened on the inside of the curve, meaning the wound would be completely different.
The murder weapon was a scythe, a much larger tool used to clear large areas of grasses.