What is the reason for the variation in the wordings of the "Forty Hadith of An-Nawawi" from their wordings in the original books such as Bukhari and Muslim, and which of them is better for memorization if there is no flaw?

The advice varies depending on the questioner's situation. If the questioner is a student of Islamic knowledge keen on acquiring it, then they are advised to begin by memorizing the agreed-upon hadiths from the book "Al-Jami' bayna As-Sahihain" or "Al-Lu'lu' wal-Marjan," then the additions from Bukhari, then Muslim, then the authors of the Sunan.

However, if they are not a student of knowledge and desire a general understanding of hadith, they can memorize "Al-Arba'in An-Nawawiyyah." The slight variations in some of its wordings do not matter because they are minor, or caused by differences in versions and narrations of the original books, or a rare typographical error. Then, they are advised to read "Riyad As-Salihin" and "Bulugh Al-Maram," and most importantly, to read extensively while consulting the commentaries.