Mobs cast shadows through blocks

Mobs standing on solid non-transparent blocks cast a shadow on blocks underneath.
See photos below.
Horses standing on blocks above me cast shadow even though blocks are solid.

 
Additional information from Skaijie:

 Steps to reproduce

1. Create two stacked platforms with a 1 block gap in between
2. Summon a horse on the higher platform
   → The horse's shadow is observed on the lower platform

The larger the shadow cast by the entity, the lower the shadow can be rendered. A cat will not produce a shadow on the lower platform using the above steps, but will produce one if the higher platform is changed to top slabs and the lower platform is moved upwards by 1 block. The lower shadow is also smaller than the higher shadow.