The reason for the 20-year gap during the Neo-Babylonian period is becaue of their beliefs about 607, but there is more going on with their alternative chronology for the kings of Judah and Israel, which has an increasing discrepancy the further back you go.
Please see here for more information about how and why the JW chronology is manipulated to suit their beliefs.
konceptual99:
Just bumping in case anyone has a chance to comment on why there are numerous sites on the net that refer to a date of 997BCE for Rehaboam's ascension?
The reason is similar to that of JWs (see link above), relating to biblical literalism overriding overlapping reigns.