A quick perusal of the internet reveals that there are at least four numbering schemes used for coumarin derivatives. In each case, elision, duplication or out-of-sequence numbering creates a gap that accounts for the odd numbering of substituents.
Skip over the fused ring carbons
Some numbering schemes do not concern themselves with the carbons shared between the rings, as seen here (Fig. 1). This is common in PAH numbering schemes and is recommended by IUPAC1.
4a and 8a
As seen here, 4 is followed by 4a and 8 is followed by 8a.
4' and 8'
Alternatively, the a modifier can be replaced with a prime, as seen here (Fig. 2).
9 and 10
This coumarin-like structure denotes the fused ring carbons 9 and 10.
(1) International Union of Pure and Applied Chemistry; NOMENCLATURE OF FUSED AND BRIDGED FUSED RING SYSTEMS; Pure and Applied Chemistry; 1998, 70, pp. 143-216