The letters Cyrillic took from Greek and the letters it supplied for Slavonic are named on different principles, and comparing the two lists tells you something about how the alphabet was put together.
The Greek derived letters carry ordinary Slavonic words: I, I know, word, good, it is, earth, who, how, people, think, ours, he, rest, say, word, firm, learning. They are common, short, and drawn from the vocabulary of worship, which is what a teacher would choose for a class learning to read out of a service book.
The added letters divide. Some also got real words: zhivete, an imperative meaning live; dzelo, an adverb meaning very; cherv, the noun for worm. Others got no word at all, only the sound with a vowel hung on it: sha, shta, yery, yu. And the yus letters carry descriptive labels, big and little, which are classificatory terms rather than names in the same sense.
The pattern suggests the naming was done in more than one pass and possibly by more than one hand. A single author working through the alphabet with one method would not produce a list that is a coherent devotional vocabulary at the start and a set of phonetic labels at the end.
It also fits what is otherwise known about the alphabet, which is that it was assembled institutionally rather than invented at a stroke. A committee leaves this kind of trace, and a single inventor generally does not.