Google PageSpeed is a family of tools by Google Inc, designed to help a website’s performance optimizations. It was introduced at Developer Conference in 2010. Wikipedia
The field data is a historical report about how a particular URL has performed, and represents anonymized performance data from users in the real-world on a variety of devices and network conditions. The lab data is based on a simulated load of a page on a single device and fixed set of network conditions. As a result, the values may differ.
Why does the performance score change from run to run? I didn’t change anything on my page!
Variability in performance measurement is introduced via a number of channels with different levels of impact. Several common sources of metric variability are local network availability, client hardware availability, and client resource contention.