A benchmark is made to stress test the component in an attempt to give you an accurate representation of what it could do.
Would you have rathered the card give it 50% and goof off the rest of the time messing up your benchmark score?
The frequency the gpu is running at and the power limit are 2 different things and are not necessarily correlated.
The gpu can be running at full frequency and not necessarily be at 100% of its power limit.
Even the reverse isn't necessarily true, I can run a card at its power limit but limit the frequency.
This limiting of frequency / power limit can be done with numerous programs, my favorite being MSI Afterburner.
http://xtrememehardware.com/wp-content/uploads/2017/12/guru3d-msi-afterburner-442-725x454.png
Having said all that running a gpu benchmarking program like unigine heaven benchmark or furmark will attempt to use all of the available gpu power and push frequencies as high as possible in order to calculate your score.
At some point the gpu will reach a maximum frequency and throttle.
This is normal and means this is highest frequency the card can perform at with your current cooling solution and power delivery.
Some games can also do this with certain settings, for instance, running Farcry 5 with ultra settings on 4k with most any setup.