Weather files will not always have the conditions expressed
by the design conditions -- which are long term perspectives of a
location.
You should also read about SizingPeriod:WeatherFileDays and
SizingPeriod:WeatherFileConditionType which use actual days out of the
weather file. The requirement behind these two is two
fold:
1) you may not have design conditions for a location and wish to use the
precalculated days (extreme summar) as your sizing basis.
2) you may wish to run a longer period than a day to get the
sizing.
But the fact is that weather files don't necessarily represent the
extremes of the location and you may want those extremes represented in
your sizing.