The First Point:
It should not be understood that “Saheeh al-Bukhaaree” and “Saheeh Muslim” encompass every authentically reported statement of Allaah’s Messenger (صلّى الله عليه وسلّم). Rather, there are many authentic Hadeeth collected by other scholars that neither one of them (al-Bukhaaree & Muslim) recorded in their collections.
“The intent of both al-Bukhaaree and Muslim was only to collect a large number of authentic Hadeeth just as a book of Fiqh  may collect texts pertaining to many different issues, not encompass every possible issue. However, if there is a Hadeeth having an authentic chain of narration that one of them may have left out, or even both of them, then they would have had a reason to do so or they may have recorded a similar Hadeeth in its place. From what is well-known from them, it may be assumed that perhaps they felt it better not to record it, or perhaps they even forgot. They also may have left certain Hadeeth out in order not to make their collections even longer than they are, or they felt what they have recorded was sufficient pertaining to a certain topic. They may have had other reasons as well.”
The Second Point:
Scholars unanimously agree and consider “Saheeh al-Bukhaaree” and “Saheeh Muslim” to be the most authentic written works of Hadeeth. Regarding this, the famous scholar, Ibn Taymiyyah, said, “There is no Hadeeth they both agree to be authentic except that it is undoubtedly authentic.” He also said, “The vast majority of their narrations, the scholars of Hadeeth know with certainty that the Prophet (صلّى الله عليه وسلّم) actually said them.”
 Fiqh: The knowledge of practical legislated Islaamic rulings by their related detailed proofs. – Shaykh Ibn al-‘Uthaymeen, “al-Usool min ‘Ilm al-Usool” pg. 7. – T.N.
Source: Biographies of the Famous Hadeeth Scholars and an Introduction to Their Collections – Shaykh Uthaymeen (rahimahullaah) | Translated by: Abu az-Zubayr Harrison (rahimahullaah)