thing is this song spans 2 octaves, which is the full vocal range of a normal person. And it's in Eason's original key. So it's REALLY hard for a female singer to sing at this key.
+Ash T Fiona sung 1 octave higher than Eason though at same key. What Xuekai means is female singer generally has a smaller vocal range. Thus it is VERY hard for a female singer to sing this song.