[LS]

I know, right? For some reason, I don’t think she would have been good in harmonies with other people. Her voice was so different. But Richard kept all the harmonies in house.

[shadowlands1960]

This music blogger wrote an article attempting to explain Karen's voice, here's his main take:

Karen normally sang as a contralto, but she had the rare ability to transition from that more typical female register to a much lower range—something she called her “basement.” Thanks to Richard’s songwriting and song selection, her basement voice was used masterfully on music that tended toward the melancholy, the “rainy days and Mondays always get me down” type of song.