Kaf writes a plain k and continues the Semitic consonant behind the Hebrew kaph, the Syriac kaph and the Arabic kaf, all four keeping the same inherited name. Beside the plain letter Ge'ez has a labiovelar counterpart, a k made with the lips rounded, and it does the same for g, for the ejective q and for the velar fricative of harm.
Those labiovelars are written as signs of their own, and they behave differently in the grid. Where a plain letter has seven orders, one for each vowel of the classical language, a labiovelar has five, because the rounding and the rounded vowels do not combine and the u and o forms are not written. A reader who takes the fidel to be a tidy rectangle of twenty-six by seven will meet these rows and find them short.
That is the practical reason the atlas records the twenty-six plain consonants as its letters rather than attempting the whole character inventory. The twenty-six are what is recited, what is taught and what the traditional order names. The labiovelars, the consonants added for Amharic and Tigrinya, the numerals and the punctuation all sit around that core, and describing them as further letters of the Ge'ez alphabet would overstate what the classical language actually used.