Hello Takahashi-san,
Please see Section 2.4.6 of the "Server Efficiency Rating Tool (SERT) Run and Reporting Rules" PDF.
The CPU worklets perform three warmup iterations before the calibration/measurement iterations. You can edit the "config-all.xml" file on the UI side to modify the number of warmup iterations run to reduce the run-to-run variability:
<interval-count id="cpuWarmupIntervalCount">3</interval-count>
Try adjusting that value higher or lower, and see if that improves your LU variability.