I think it is related to this:
The following quotes are from Chris’s answer:
This error is not actually that severe in terms of the consequences for energy use. Your overall energy use values that you get out of the simulation should still be pretty accurate. However, if you are looking at surface temperatures or sub-zone results, these values will likely not be accurate.
So it depends on what you are trying to achieve. You likely have “concave” zones in your model:
The error results from the fact that you have zone geometry that is concave (or L-shaped to put it more simply) and this means that Energy Plus’s simplified algorithm for ray tracing sun rays from the windows into the zone is not working so well (since some of the rays intersect more than one wall of your zone).
To solve it:
So, to solve the problem, you can either not look at surface results, run the simulation without interior reflections, or break up your L-shaped zone into two zones with an air wall between them.
Does this help?