Shabbos 28:18

Continuing from the previous halacha about accepting a later surveyor's larger measurement, the reason we do so is because we can assume that the earlier surveyor made a mistake and measured the 2,000 cubits diagonally from the corner of the city, reducing the measurement and making the distance between the city and the edge of the Shabbos boundary less than 2,000 cubits. The later surveyor measured the 2,000 cubits from the side of the city, yielding a larger and more accurate measure. We do not, however, assume that the first surveyor might have made any mistake greater than this. Therefore, if the second measure is less than 580 cubits greater than the first measure (about 870 feet), we accept it, but not if it is larger than this.

Shabbos 28:19

Even a male or female servant is believed when they say "The Shabbos boundary extends this far." An adult is believed if he says, "We would proceed this far when I was a child." The reason his testimony is accepted in such a case is because the Sages said that lenient standards should be applied in such situations because the 2,000 cubit limit is of rabbinic origin.