modern multicore CPUs (both AMD or Intel) can go above the normal frequency when some cores are idle. This is adaptive behavior because the CPU is monitoring the power usage: if power usage is low and temperature is low, the frequency can be increased a bit to let some cores go at higher frequency.
The frequency will return to standard when all cores are active (there are threads running and not waiting for completion of an external I/O or interrupt).
There are tools that display not just the maixmum frequency, but the frequency of individual cores and you can also see that in Windows Task Manager your CPU is not used at 100%: is a single thread is busy at 100% on an 8-core CPU, the total CPU usage is about 15%, power consumption is still low, and the single thread can be boosted to run at higher frequency.
PBO is for AMD processors. For Intel, this kind of boosting is named "SpeedStep" (it acts not only on frequency but also voltage, plus transition of power modes, and insertion of wait cycles in case of spike of temperature or other abnormal conditions, such as difficulty to synchronize/coordinate bus transfers and priorities or acknowledging the transfers, or detection of bus transfer errors, or unexpected timeouts requiring frequent resets and resynchronizations).
Some mobos also allow doing the same for the bus bridges, caches, RAM, or various other devices; display boards have similar tuning (in their drivers) for their GPU shaders, their GDDR RAM controler, using their own thermal and power monitors.
Real "overclocking" occurs when you start running the CPU with higher power than normal specs (and is allowed on some series of non-mobile processors, provided you use above-standard cooling systems and power sources and some custom software tool running to monitor power and temperature more agressively with complex dynamic tuning rules). Normal conditions use monotoring at much lower rate with more tolerance in delays for adapting rates and voltage, using much simpler rules.