Cyrillic buky is the point at which the arithmetic of the two Slavonic alphabets parts company for good. Glagolitic counts its own letters in sequence and gives buky the value two. Cyrillic, keeping the Greek numeral values, has nothing to give it: there is no Greek letter in that slot, because the Slavic b is exactly the sound Greek lacked, so the Cyrillic letter is passed over in the counting and carries no numeral at all. Every value after this one is displaced between the two scripts, and the displacement grows as more Slavic-only letters are passed over.
That difference is a practical tool. A number written out in letters in an early Slavonic manuscript can often be read only one way, and reading it the other way gives a different figure, so a numeral in a text of uncertain background can indicate which system the scribe had been trained in. Where a Cyrillic manuscript preserves Glagolitic arithmetic, the usual explanation is that it was copied from a Glagolitic exemplar and the numbers were carried across unconverted.
The names match in both scripts and the sound matches, which is the pattern across the whole alphabet: the two scripts share a language, an order and a set of names, and share very little of their appearance.