I'm not sure, but I guess it's just the article changing according to the first letter of the following noun. "Ka" in front of k becomes "Ke": ka hale, ka noho, ka tūtū kāne (grandpa); but ke keiki, ke kaikamahine, etc.
"Use ke when the noun that follows begins with the letters K, E, A, or O. Use ka with almost all others! This is commonly referred to as the KEAO rule." - from the "tips" that show on the website but not the app for some bizarre reason