A score UNDER 100% means that your motherboard is UNDER reporting your power consumption, which will make the CPU speed up, increasing the voltage and temperature.
Ryzens boost according to 3 values. Temps, power and loads.
Assuming you are like most gamer-type performance seeking ppl, you'll have a decently beefy cooler, usually capable of well more capacity than the cpu could demand. So temps are rarely a major issue, comparatively.
Assuming you run Cinebench or Prime95 or other constantish high load.
That leaves Power. If the motherboard reports an accurate number to the cpu, anywhere close to 95%+, your boosts will hit an imposed limit when it reaches 65w, since that's the AMD set limit for the 5600x. Your all core boost might be 4.2GHz at that point because the cpu won't break that set limit.
Assume the motherboard instead reports 50% power use to the cpu. You might be already at 65w and 4.2GHz all core, and have decent temps, so the cpu now has room to try higher power use, higher clock speeds, and you get 4.6GHz boost instead. In reality you are pushing 140w. Looks good to the average person, who wouldn't prefer 4.6GHz all core to 4.2GHz, and the increased Cinebench scores, but that's on a 5600x. Imagine doing the same to a 5950x. You'd be looking at VRM's hitting power ceilings, burning out possibly as in reality that cpu is pulling over 300w but the motherboard is only reporting 142w.
And then you enable PBO, raising power limits and decide you have room to OC. What initially looked like an advantage and plenty of headroom suddenly evaporates.