Renoise indeed does show a different CPU than task manager. It shows the “relevant” CPU usage, the CPU usage which currently is used in order to calculate audio in real-time.
In other words: how much CPU power in your current setup currently is used in order to be able play back the song. Everything else would be misleading because it’s simply not relevant for real-time audio processing.
Lets say you have 8 CPU cores and are playing back a single VST instrument with all 8 cores enabled.
In this case only one core will actually be used (assuming the VST does no internal multi-core processing - which is the common case).
If you now see a CPU usage of 25% in Renoise this means:
The song (the single VST) is calculated 4 times faster than real-time. 200% would mean that twice the time is needed to calculate the VST’s sound, which then results into crackles because stuff gets played back faster than it can be “delivered” to the audio stream.
If you now add more and more plugins to this song, the other cores will start processing those VST too - in parallel, instead of letting one core, one CPU do all the job.
If you for example add 7 more VST, which all only take up to 10% of CPU power each, you still only have 75% of power left in order to play back the song because the first one still blocks real-time processing most.
So the core which needs longest time to do it’s job is what’s interesting. How things are currently splitted among the cores, and not the sum of all core loads.