I think you're misunderstanding what's being shown in the plot. If you look at the actual data, Sydney barely existed as a name for either gender for a long ti

I think you're misunderstanding what's being shown in the plot. If you look at the actual data, Sydney barely existed as a name for either gender for a long time. Then it became a very popular female name (top 25), while still barely existing as a male one. To illustrate: in 1960 there were 128 female Sydneys and 52 male. In 2000, there were over 10k female Sydneys and 126 male.