Why at 1 Corinthians 10:9 does the Jehovah's Witnesses NWT use the word "Jehovah" when the Greek uses "Christos/kurion" referring to Jesus Christ?

1 Corinthians 10:9, "Nor let us try the Lord, as some of them did, and were destroyed by the serpents." In context the Apostle Paul is referring to Israel at Verse 7, "And do not be idolaters, as some of them were; as it is written, "The people sat down to eat and drink, and stood up to play. (Exodus 32:4.) So why does the Apostle Paul "juxtapose" or contrast God the Father with Jesus Christ as being one and the same God? 1 Corinthians 8:6, "yet for us there is but one God, the Father, from whom are all things, and we exist for Him; and one Lord, Jesus Christ, by whom are all things and we exist through Him."