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 the long-term average (the 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/