This is confusing because the underlying manuscript tradition is confused.
First, Abijam and Abijah are variant spellings of each other and the record of the kings of Israel and Judah do not have uniform spellings of individual's names. (There are numerous examples of this.) Most modern English version make the spelling uniform which means they have to choose one of the various spellings of a name.
In the case of Abijam/Abijah we have the following for Rehoboam's son in 1 Kings 14:31, 15:1, 3, 6, 7, 8
- אֲבִיָּ֖ם = Abiyam for most Hebrew manuscripts
- אֲבִיָּ֥ה = Abiyah for some Hebrew manuscripts, the LXX (Greek) and the Hebrew & LXX of 2 Chron 12:16, 13:1.
Thus, the Hebrew witnesses are inconsistent and contradictory. Many versions either attempt to reflect this inconsistency of make the spelling uniform. If the latter choice is made, they must make the choice about which spelling to adopt. Most provide a footnote to explain. The NIV has chosen to unify the spelling with "Abijah".