Most likely because they took this text from different manuscripts that have a slightly different wording and perhaps also to understand the Middle-Eastern culture from which Jesus came. Some manuscripts have the Greek ouk (not) as opposed to oupo (not-as-yet), though the manuscripts that contain oupo are of earlier origin and were discovered later. Oupo is suppoted by these major manuscripts (Sinaiticus, Vaticanus, etc.) as well as the scholarly compositions of the various manuscripts (i.e. Scrivener, Wescott-Hort, Stephanus).
EquusNomVeritas made a good contextual argument even in the case of ouk. If one reads the whole chapter, it is clear that the thematic issue is timing.
However, we must also remember that Jesus was operating in the framework of a Middle-Eastern culture, where it is considered polite to initially refuse an invitation and shift the onus to the host to offer again (typically the third offer is considered the genuine offer, and the “real” answer is given then). This brings to mind another “proof-text” for Jesus’ lack of truthfulness, Luke 24:28, where he displays the culturally polite method of refusal. This time, however, the disciples “strongly urge” him to eat with them, most likely indicating repeated asking until Jesus finally gives his assent on the third and genuine offer.
This shows the danger of proof-texting without an understanding of grammar, context, and culture. Sorry to distract the thread, I was just trying to bring some clarity to AnAtheist’s question.