Source : weather.gov
This is mentioned in the book I have been reading regarding stability in atmosphere. But I am not able to pin point exact reason behind this.
"As the layer subsides, it becomes compressed by the weight of the atmosphere and shrinks vertically. The upper part of the layer sinks farther, and, hence, warms more than the bottom part." (Ahrens et al.,2019)
-Ahrens, C.D., Henson, R. (2019). "Meteorology Today: An Introduction to Weather, Climate, and the Environment". 12th Edition. Chapter 6 ( Stability and Cloud Development), CENGAGE Learning Custom Publishing, page 146-147