When generating TMY data, we try to select months in such a way that annual sum of GHI/DNI values in the TMY file is consistent with the annual average calculated from time series. However it may not be possible to find representative months where sum of irradiation as well as meteo values will equal the long-term average. Therefore, we may be required to slighlty adjust the meteo values to maintain similar averages as calculated from time series.

More information on how SolarGIS TMY data is prepared can be seen here: http://geomodelsolar.eu/_docs/papers/2014/Cebecauer-Suri--SolarPACES2014--Improved-method-for-generating-Typical-Meteorological-Year-data-for-solar-energy-simulations--presentation.pdf