Bell family mystery about Robert Bell Sr.'s parents

I'm a fifth great-grandaughter of Robert Bell, Sr. (b 1753 Sussex, New Jersey, d 1837 Richland Ohio), and I'm having a little trouble with a Bell family mystery.  All the records
I can find show that Robert Bell Sr. was the son of Isaac Bell and
Keziah Corwyn, but Keziah Corwyn was considerably younger than Isaac
Bell, and in fact was born AFTER the recorded birth dates of some of
her children, including Robert Sr. I can't find any records of
another wife for Isaac Bell, but there must have been, unless the
records I've found so far are totally wrong about Keziah Corwyn's birth
date or the birth dates of her husband and children. Or maybe it was a different Keziah Corwyn.  Can anyone shed any light on this for me?

Thanks,
Marian Bumala nee Yearian