Cyrillic zemlya writes the same consonant from the same place in the order, and it is numbered seven where Glagolitic numbers nine. Seven is the value of Greek zeta, which is the Greek letter this slot answers to, so the Cyrillic figure is inherited and the Glagolitic figure is counted.
The offset between the two systems has now grown to two, and it is worth seeing exactly how it grew. Two Slavic letters earlier in the order, the sign for b and the sign for zh, have no Greek counterpart at all. Glagolitic numbered them anyway, because it numbers everything; Cyrillic passed over both, because Greek had nothing to pass on. Each skipped letter widens the gap by one, and the gap will keep widening at every further Slavic-only sign.
The names match, zemlja beside zemlya, differing only in the ending that the various abecedaria record variously. The sense is the same word for earth in both.
For a reader the practical point is that a number in an early Slavonic text cannot be converted between the scripts by transliterating the letters. The letters carry different values, and a figure copied across without conversion produces a wrong number that looks entirely normal, which is one reason numerals in these manuscripts are edited with care.