Appendix 3A in the NWT said the following;
"Greek Text: In the late 19th century, scholars B. F. Westcott and F.J.A. Hort compared existing Bible manuscripts and fragments as they prepared the Greek master text that they felt most closely reflected the original writings. In the mid-20th century, the New World Bible Translation Committee used that master text as the basis for its translation. Other early papyri, thought to date back to the second and third centuries C.E., were also used. Since then, more papyri have become available. In addition, master texts such as those by Nestle and Aland and by the United Bible Societies reflect recent scholarly studies. Some of the findings of this research were incorporated into this present revision.
Based on those master texts, it is evident that some verses of the Christian Greek Scriptures found in older translations, such as the King James Version, were actually additions made by later copyists and were never part of the inspired Scriptures. However, because the verse division generally accepted in Bible translations was already established in the 16th century, the omission of these verses now creates gaps in the verse numbering in most Bibles. The verses are Matthew 17:21; 18:11; 23:14; Mark 7:16; 9:44, 46; 11:26; 15:28; Luke 17:36; 23:17; John 5:4; Acts 8:37; 15:34; 24:7; 28:29; and Romans 16:24. In this revised edition, those omitted verses are indicated by a footnote at the location of the omission.