From the Wikipedia article about the Epic of Gilgamesh:
From the diverse sources found, two main versions of the epic have been partially reconstructed: the standard Akkadian version, or He who saw the deep, and the Old Babylonian version, or Surpassing all other kings. Five earlier Sumerian poems about Gilgamesh have been partially recovered, some with primitive versions of specific episodes in the Akkadian version, others with unrelated stories.
It seems that the standard Akkadian version was discovered all in one place, at the ancient Library of Ashurbanipal, and it was clear that all of these tablets were part of a single unified story. About the Old Babylonian version, by contrast, Wikipedia says:
This version of the epic, called in some fragments Surpassing all other kings, is composed of tablets and fragments from diverse origins and states of conservation. It remains incomplete in its majority, with several tablets missing and big lacunae in those found. They are named after their current location or the place where they were found.
... and then goes on to list various tablets found in ones and twos at diverse locations (the Pennsylvania tablet, the Nippur school tablet, the Tell Harmal tablets, the Ishchali tablet, the Sippar tablet, etc.), many of which roughly overlap with parts of the story told in the standard Akkadian version.
What is the evidence that all of these scattered tablets are parts of a unified whole? In other words, why do they talk about an "Old Babylonian version" rather than fragments of what might be various different versions in contrast with the single long Akkadian version?