1 Cor. 8 "For though there be that are called gods, whether in heaven or in earth, (as there be gods many, and lords many,)"
You may believe in only One True God and Lord. But others may use the terms to refer to a different Person (or person).
In fact Muslims do refer to Allah as Lord: "Thy Lord is self-sufficient, full of Mercy: if it were God's will, God could destroy you, and in your place appoint whom God will as your successors, even as God raised you up from the posterity of other people." Qur'an, Sura 6