i think :
- on mtv japan she did that because the last tone when it said "kimi ja nakya dame de" the 'de' in the original version is remixed... maybe she did that to make the performance as much similar as the original version...
- in fly high or other performance.. i think she did it sometimes to prevent missnote.. i think
-otherwise... she never does lip sync
she sings with her own voice ^^