I have taught a South African Chinese girl whose given names were Nikita Bonnie. Also a white South African girl, at another school. I suspect that the -a ending makes people think it's feminine, as you say, and it's also a welcome change from all the Nicole little girls without moving too far away from Nicole/Nicola. I rather suspect also that the Elton John song might mislead people into thinking he was singing to a woman rather than a man ... that would amuse him, I think!