The letters at the end of the Slavonic order that write a glide followed by a vowel are worth taking together, because as a group they show what kind of design decision was being made. The language had a series of such sequences, and there were two ways to write them: with two signs, a letter for the glide and a letter for the vowel, or with a single sign for the whole combination. Both Slavonic alphabets chose single signs for most of the series.
That choice has consequences a modern reader can see. It makes the alphabet longer than it strictly needs to be, and it makes the relationship between spelling and sound more direct, because a reader meets one shape for one syllable rather than assembling it. It also means the alphabet encodes a piece of Slavonic phonology in its inventory: the sequences it gives letters to are the ones the language treated as units.
The Cyrillic alphabet made the same choice for the same sequences, and in one case, the glide with e, it built its sign visibly out of two letters written together, which shows the alternative being used alongside the rule. The atlas pairs the counterpart letters of the two scripts where a pairing can be made and marks the ones where it cannot rather than forcing a match.