in talking with fans, that is actually some of the gripes and problems [[again according to the fans) between Jean and mary. Jean was in the studio a LOT, often M and C were replaced in the backgrounds, Jean was doing nearly all of the leads live on tv and in concert and having to learn that material. Meanwhile mary did some occasion background work and basically sang 1 song again and again in concert. Jean was also salaried and not earning royalties on Ladder and some of the other early tunes.

So you have this problem where on one side you have Mary - stepping up as the original and trying to lead the direction of the group and then the other side Jean - a hugely talented woman who's doing the lion share of the work, getting less money and not able to really take the group in a new direction