Genesis 8:5
3 The waters kept receding steadily from the earth, so that they had gone down by the end of the 150 days. 4 On the seventeenth day of the seventh month, the ark came to rest on one of the mountains of Ararat. 5 The waters kept on receding until the tenth month. On the first day of the tenth month, the tops of the mountains became visible. 6 At the end of 40 days, Noah opened the window he had made in the ark 7 and sent out a raven; it kept flying back and forth until the waters had dried up on the earth.
LXX Genesis 8:5
3 And the water, as it was proceeding from the earth, was giving way; after fifty and one hundred days the water was giving way and diminishing. 4 And in the seventh month, on the twenty-seventh of the month, the ark settled on the mountains of Ararat. 5 Now the water, as it was proceeding, was diminishing until the tenth month; then in the eleventh month, on the first of the month, the tops of the mountains appeared. 6 And it came about after forty days that Noe opened the window of the ark that he had made 7 and sent out the raven to see if the water had subsided, and after it had gone out it did not return until the water was dried up from the earth.
Notes and References
"... Sometimes the changes to the original narrative are 'corrections' based on logical inferences arising from the translator's reading of the text. Such is the case in Genesis 8:5 which describes the scene as the waters of the flood abate ... The LXX translator has interpreted the verse to mean that the level of the water diminished throughout the whole of the tenth month before it reached the tops of the mountains - i.e., at the beginning of the eleventh - despite the fact that the Hebrew specifies that this took place a month earlier ..."
Hiebert, Robert J. V. Translation Technique in the Septuagint of Genesis and its Implications for the NETS Version (pp. 76-93) International Organization for Septuagint and Cognate Studies, Vol. 33, 2000