You gave the answer yourself:

Quote - it is as if another bodypart is fixed in space

For most static poses, the Body actor remains fixed in space (although a Pose can translate it).

When you apply a Pose, the Hip may be translated (moved) and rotated with respect to the Body. The Thighs may be rotated with respect to the Hip, the Shins with respect to the Thighs, and the Feet with respect to the Shins.

This is why the feet will end up in different positions in 3D space, and an animation between the poses will slide them between these two positions.

"If I were a shadow, I know I wouldn't like to be half of what I should be."
Mr Otsuka, the old black tomcat in Kafka on the Shore (Haruki Murakami)