Using high performance power plan? If not it'll clock down when not under load, which is why it shows the 3.8ghz as its base speed, like mine shows 4ghz.
Also shut off amd cool n quiet and any other fancy power/performance control(s). SiSandra always complains my cpu is not accurately benchmarked because of variations in speed due to turbo mode etc.; some of these controls are in BIOS/UEFI some are OS or even utilities.