Insolation results vary from Spring to Fall (sun path same)

[enerGwizz]

I am running an insolation study on a floor grid inside an east-facing space during the solstices and equinoxes.. The summer and winter results make sense, but the spring and fall results vary dramatically when the sun path is almost identical between the two. The spring has the lowest average heat gain of the 4 seasons while the fall has the highest (180 vs. 546 Btu/h average). The building geometry and insolation calculation are exactly the same. It is only the date I am changing each time.

Is there something that Ecotect is taking into account that I'm not understanding? Is there a reason these results should be that different when the sun path is almost identical? I can't pass these results onto our client without understanding them, and I think that Ecotect is doing something very strange...

Thanks in advance for any input.

[kjellanderson]

The weather file you are using contains hourly data composed into a typical meterological year (TMY). This is real, measured direct and indirect solar throughout the year. In seattle, for instance, we have cloudy springs and slightly more sunny falls. Does that answer your question?