The difference might come down to the purpose of each book. Genesis is a literal account and since Abraham was without wife when he bound Keturah to himself--she became his wife, performing the duties of a wife.
Chronicles on the other hand is a historical record and perhaps a legal document for things such as inheritance through genealogies and also the royal lineage. So while being a technical difference between words that can mean essentially the same thing, when it comes to inheritance the difference becomes quite relevant.
This assertion is my own, but to cite a source that backs up what I say about Chronicles having genealogies gotQuestions summary of Chronicles is suitable.
Another site to give perspective is from Bible.org
When producing the Septuagint, the translators divided Chronicles into
two sections. At that time it was given the title, “Of Things
The books of Chronicles seem like a repeat of Samuel and Kings, but
they were written for the returned exiles to remind them that they
came from the royal line of David and that they were God’s chosen
people. The genealogies point out that the Davidic promises had their
source in those pledged to Abraham that He would make him the father
of a great nation, one through which He would bless the nations.
As well as,
This book also taught that the past was pregnant with lessons for
their present. Apostasy, idolatry, intermarriage with Gentiles, and
lack of unity were the reasons for their recent ruin.
First Chronicles naturally divides into four sections: (1) The
Genealogies or the Royal Line of David (1:1-9:44); (2) the Rise of
David or His Anointing (10:1-12:40), (3) The Reign of David
(13:1-29:21), and (4) The Assession of Solomon and the Death of David