When selecting months for TMY P50 generation, we try to choose months from multi-year time series that are close to the monthly averages.

When selecting months for TMY P90, the criteria is that yearly sum should be a conservative value. However all months in the TMY P90 dataset need not have lower values than the long term average monthly value.

Indeed in real situations if we look at real data with low yearly irradiation conditions, it is almost impossible to find a year when all months are lower than long term average (only exception can be year following huge volcano eruptions such as the Pinatubo eruption).

More information about the Solargis TMY methodology can be seen here: http://solargis.com/support/knowledge-base/methodology/tmy-generation/