#1
the accepted name according to GBIF is
Xanthogramma festiva (Linnaeus, 1758) and the synonym is
Chrysotoxum festivum (Linnaeus, 1758). That can only be a mistake?
https://www.gbif....es/4516839
#2
CZN 2001: Opinion 1982 (Case 3090): Musca arcuata Linnaeus, 1758 and M. festiva Linnaeus, 1758 (currently Chrysotoxum arcuatum and C. festivum) and M. citrofasciata De Geer, 1776 (currently Xanthogramma citrofasciatum) (Insecta, Diptera): specific names conserved by the designation of neotypes for M. arcuata and M. festiva. Bull. Zool. Nom. 58: 241-242
can probably explain this case. Enjoy, to me this case is outside of interest.