אָמַר רַב עַמְרָם לְפִי שֶׁאֵין לְמֵדִין מִשִּׁיטָה אַחֲרוֹנָה Rav Amram says: It is because one may not learn any new details from the final line of a document. There is a concern that the holder of the document may have written in an extra line of text to his advantage, between the text and the witnesses’ signatures. Therefore, any new information contained in the last line is disregarded. Since the final line is disregarded, it is necessary that the final line of a document contain only a review of what is already written in it.
אֲמַר לֵיהּ רַב נַחְמָן לְרַב עַמְרָם מְנָא לָךְ הָא אֲמַר לֵיהּ דְּתַנְיָא הִרְחִיק אֶת הָעֵדִים שְׁנֵי שִׁיטִין מִן הַכְּתָב פָּסוּל שִׁיטָה אַחַת כָּשֵׁר Rav Naḥman said to Rav Amram: From where do you know this? Rav Amram said to him: As it is taught in a baraita (Tosefta 11:1): If one writing a document distanced the witnesses’ signatures two lines from the text of the document, leaving two lines blank, the document is not valid, as it would be possible for the holder of the document to fill in false information in those lines. But if one distanced the signatures one line from the text, the document is valid.
מַאי שְׁנָא שְׁנֵי שִׁיטִין דִּלְמָא מְזַיֵּיף וְכָתֵב שִׁיטָה אַחַת נָמֵי מְזַיֵּיף וְכָתַב אֶלָּא לָאו שְׁמַע מִינַּהּ אֵין לְמֵדִין מִשִּׁיטָה אַחֲרוֹנָה שְׁמַע מִינַּהּ Rav Amram analyzes the baraita: What is different in the case of two blank lines that that the document is not valid? If you say that perhaps the holder of the document will take advantage of those two blank lines, and forge unverified information and write it in those lines, then in the case of one line left blank as well, he can forge information and write it in that line. Rather, must one not conclude from the baraita that one may not learn any new details from the final line of a document? Therefore, only when two lines are left blank is there a concern that false information would be added to the penultimate line. The Gemara affirms: Conclude from this baraita that it is so.