Are you really surprised?
This is a common problem of stencils:

[manual]
Dynamic shadows are not clipped by walls. A shadow can be cast on a wall, at the back side of the wall, and on the floor behind the wall at the same time. Take care that level and models are very dark in unlit parts (or use the cast flag) to cover or suppress unwanted shadows.
Stencil shadows are also visible on model polygons that point away from the light source, which can lead to undesired flickering shadow polygons. To avoid this, use a material with low ambient and a high diffuse part.
[/manual]