Hello Juan,
By convention, weather file data is recorded according to local time such that weather readings are recorded on the hour (for hourly files) or averaged over the hour to coincide with actual clock time. Clock time is almost always different from solar time because any given location is not (usually) exactly along the standard meridian. SAM's simulation engine runs off of local weather file time, and calculates sun position according to the midpoint of the each hour. Often, this leads to asymmetries in things like cosine loss, peak sun position, morning/afternoon production, etc.
Regards,
Mike Wagner
NREL