Not sure if this is universal, but when my wife and I were thinking about naming our son, we observed that in Spain there seems to be much more variety of girl than boy names.
In fact, you can almost prove it by bijective mapping to a subset. Almost all typical boy names have a feminine version (Juan->Juana, Carlos->Carla/Carlota, Manuel->Manuela, Adrián->Adriana, and so on... there are some exceptions, but not many) while the opposite is not true, there are plenty of common girl names without masculine version (Laura, Marta, Elena, Carmen, Sara, and many others).
There are also papers doing the actual stats and concluding that girl names are more diverse. No idea what the reason is.
In fact, you can almost prove it by bijective mapping to a subset. Almost all typical boy names have a feminine version (Juan->Juana, Carlos->Carla/Carlota, Manuel->Manuela, Adrián->Adriana, and so on... there are some exceptions, but not many) while the opposite is not true, there are plenty of common girl names without masculine version (Laura, Marta, Elena, Carmen, Sara, and many others).
There are also papers doing the actual stats and concluding that girl names are more diverse. No idea what the reason is.