How could the cuneiform sign for land or mountain have both Sumerian and Akkadian readings?

The cuneiform sign for "land" or "mountain" (originally a pictograph of three mountain tops) could have both Sumerian and Akkadian readings because the Akkadians adopted the existing Sumerian writing system and adapted it to their own distinct language.1

When the Akkadians incorporated these signs, they retained the original Sumerian logograms but pronounced them as the corresponding Akkadian words. Consequently, the sign carries the phonetic value kur based on the Sumerian term, while also acquiring the values mat and šad from the Akkadian words mātu ("land") and šadû ("mountain"). This process of assigning multiple phonetic values to a single sign, known as polyphony, became a common feature as the script was adapted across different cultures.12