Two-source hypothesis
There is something that is called minor agreements. These are parts of the text, where both Matthew and Luke agree, but where Mark does not. To explain this,  many people believe that Matthew and Luke did not use the text of Mark as it was included in the New Testament, but rather an earlier version. Some people even say that they did not use the same version of this earlier text. This could also explain why the text in Mk 6,45-8,26 is missing from Luke.