I’m not certain why this has to be known. Whatever…
The English transliteration “Larob” is found in Genesis 48:16 and means multitude, abundance, numerous etc, deriving from the Hebrew root word, a verb, Rabab: “To become many”, which tends to define multitude better than the following rendering in verse 19
The English transliteration Melo in Genesis 48:19 means multitude, fullness, contain, etc, deriving from the Hebrew root word, a verb, Mala: “To be full, to fill”, which tends to emphasise fulfilling (in numbers?)
Both are typical of any given single Hebraic word which gives a variant range of meaning within different context, but in essence, the meanings are similar. However, in these two contexts of verse 16 and 19, both Larob and Melo; when they “met in the middle“ in Genesis 48 in the Septuagint, it seems that they end up having the same sense, at least to the Greek speaking scribes of the LXX that is, who seem to have sensed that both of the Hebrew words have the same meaning, and the scribes went ahead and translated both Hebrew words in Genesis 48:16,19 with one Greek word pletqov, (increased, multiplied etc)