Plasma panels age. The last models were rated for 60,000 hours lifespan (to lose half of their brightness). So if they measured some 10-15 years old panel that

Plasma panels age. The last models were rated for 60,000 hours lifespan (to lose half of their brightness). So if they measured some 10-15 years old panel that spent most of its life being turned on, there's your answer.