A normal house roof in Calgary will have at least 2 panels (sections), and most often many more.  Each of these panels will face towards different points of the compass.  This means that weather will affect each panel differently, and the roofing installation will weather differently on its various parts (read about that here).  While the variation in weather and sunlight may be slight, patterns become evident over the course of years and decades and wear will become more evident in some areas than in others.

In Calgary, north facing parts of the roof receive the least amount of sunlight.  East facing parts of the roof receive more sunlight but are cool from the night time.  The south and west parts of the house will receive the most solar energy.  The roof of a house will receive more energy per square foot because the panels are more directionally opposed to the sun.  While this energy is intense, it is increased markedly by walls and features.  The lower parts of a roof directly adjacent to a wall or window can see an increase of 50% or more solar radiation than the upper parts, and shingles in these areas can begin to deteriorate faster as a result.  Sometimes ventilation in these areas is poor and heat can build up to undesirable levels, which only serves to compound the problem.
It is not uncommon to see ledges on a house that are a “season ahead” when compared to the rest of the roof – the wear and tear these areas experience is accelerated and gravel loss will be evident and widespread.