我也不太明白，不过搜索了一下，区别在这里
A pitaya /pɨˈtaɪ.ə/ or pitahaya /ˌpɪtəˈhaɪ.ə/ is the fruit of several cactus species. "Pitaya" usually refers to fruit of the genus Stenocereus, while "pitahaya" or "dragon fruit" always refers to fruit of the genus Hylocereus.
好像这意思就是植物的属不同