Actually it is uniform…along the top and the bottom.
You apparently have gable vented louvers…which probably is trapping the heat along
the eaves while at the same time stagnant air or no cross ventilation existing is causing the shingles near the top to deteriorate…if not for the additional layer under the ridge caps I suspect they too would be deteriorated.
Organic shingles also tend to blister more than your fiberglass shingles…taking into consideration there is venting problem (condensation etc) I am sure this also has contributed to their failure.
Is it a true vault or are they scissor trusses?
Also was that the east side of the house, how the the other side of the roof look?
The roof looks likes it about 15 - 20 years old but then again its hard to tell sometimes. I just inspected a roof that had 3 tab shingles that have last over 30 years…I was amazed at their condition. If not for the poor venting and numerous hail storms we have had the owner could have probably gotten another 5 years or more.
Any pictures of the interior…including infrared on the ceiling?