Transmute Rock to Mud

[Prism]

This spell turns natural, uncut or unworked rock of any sort into an equal volume of mud. Magical stone is not affected by the spell. The depth of the mud created cannot exceed 10 feet. A creature unable to levitate, fly, or otherwise free itself from the mud sinks until hip- or chest-deep, reducing its speed to 5 feet and causing a –2 penalty on attack rolls and AC. Brush thrown atop the mud can support creatures able to climb on top of it. Creatures large enough to walk on the bottom can wade through the area at a speed of 5 feet.

The spell suggests that anyone caught in the mud is reduced to a speed of 5' assuming they cannot free themselves. The mud is typically 10' deep. But then it states that creatures that can walk on the bottom can move at a speed of 5'. If any creature can move at 5' what is the need for this comment? Or does it mean that creatures that cannot walk on the bottom cannot wade at 5' and therefore cannot move at all?

 

[Ferret]

It means even if you are 12 foot tall you are still limited to 5ft. movement, even tohugh you touch the ground, where as the chest-high is limited to swimming and has 5ft. movement.