I noticed something similar in a 1999 Malibu we once owned. My shop guy explained this was done to keep the liner from rattling against the roof as the vehicle ages. He didn't expect to see any sagging for the expected life of the car. We owned that one for 8 years and never saw a problem. Trunk liner in the Malibu was a different story, but it got wet a number of times so that was expected.