It is just the bias introduced by the cache. This is a debug rendering with a radius of 10cm, focus your attention on the circled areas:
and this is the same rendering with a radius of 1cm and 10x more photons traced (still 1secs pre-processing time):
As you can see, the "A" area is about the same while the "B" area is brighter with a smaller radius. This happen because a circle of radius 10cm placed on "B" area will include empty space but the values is still divided circle surface so the result is darker. Indeed, smaller is the radius and less error there is.
Note how this doesn't happen on "A" area because the 10cm circle is fully filled by the surface and fully bombarded by photons.
@lacilaci, this is also the reason why using a 90deg angle is brighter, instead of empty space, you include photons hitting the perpendicular wall.
For reference, this is a rendering with LIGHTCPU (the true reference when doing forward ray tracing, PATH is backward):
About the same of the rendering with 1cm radius.
Short version: larger is the radius more and more bias (aka error) you have.
simplified.zip
