Cyrillic glagol writes the same consonant, sits in the same fourth place and is numbered three, against the four that Glagolitic gives it. The gap of one is the same gap that opened at the previous letter, and it comes from the same cause: Cyrillic follows the Greek values, gamma is the third Greek letter, and the Slavic b in second place takes no number in that system.
The names are close, glagoli beside glagol, and both continue the Slavonic verb glagolati, to speak. The Glagolitic form is the imperative and the Cyrillic form is a noun, word or speech, so the same root is being used in two grammatical shapes to name one letter in two scripts. Whether that difference is original or the product of centuries of recitation is not recoverable from the surviving lists.
Only Glagolitic took its name from this letter. Cyrillic is named for Constantine-Cyril, whom the Slavonic sources credit with making an alphabet for the Slavs in the 860s, and the reasons for attaching his name to the later of the two scripts rather than the earlier are a matter of how the tradition developed in Bulgaria after the Moravian mission collapsed. The name and the object it now denotes do not necessarily belong together.