Character Song Question

Some of the best songs from KimiUso are the "original" songs, or character songs. However, I noticed that if you listen to the ones on the official soundtrack, and then listen to the same song when it was released with the Bluray sets, they sound different, like they're being sung by someone else. Look at Kaori's song, My Truth ~Rondo Capriccioso~, and Tsubaki's For You ~ Tsuki no Hikari ga Furisosogu Terasu, on both the soundtrack and Bluray cd. The Bluray set can be found here: http://hikarinoakariost.info/shigatsu-wa-kimi-no-uso-original-song-vol-1/.