How do you explain the thermal resistance decrease?

The material's thermal resistance can be slightly affected by the specimen preparation's volume and contact surface area when applying the pressure.

In the test, it may also have some small air voids inside of the putty samples in these two test phases.

The difference can be measured due to the tolerance in its nature.

Based in the test results, it can indicate the material's performance can keep in a stable state after aging test and no remarkable outgassing issue exists during the test.

Please let me know if you have any further concerns. Thank you!