My guess is...
Really poor guess. My disappointment is that it models reality poorly. The same goes for TSS.
NP is used to translate a variable effort into a pseudo steady-state effort... which is then used to determine training stress. NP and TSS are fundamental for tracking your training with power.
I've found TSS to be a poor gauge of training stress for me. Mostly because it greatly understates the stress of true 100% efforts. There is a world of difference between a 95% and a 100% effort, that is missed by the TSS value.
The reason for using CP values *instead* of the standard NP formula is because it could more accurately represent the effect of variable efforts.
Again Bike Score was built around the same thinking by Phil Skiba.http://www.physfarm.com/bikescore.pdf
"Although it remains to be seen whether the above changes represent a substantial (indeed
any) improvement over Dr. Coggan’s rather robust solution, we suspect that more formal
studies will demonstrate no signiﬁcant difference between these two different solutions."
It does sound like Phil is waiting to release some more stuff soon though so maybe you will have a better model.
You also say there is a world of difference between 95% and 100%. Mentally sure, physiologically probably not that much. Energy wise, 300w CP, 25kJ W'. Flat 1000w sprint effort you would use your W' up in 36 secs thats 100%. 95%w over CP would be 965w or for time what a second less or using only 23.7kJ. You are taxing all of your systems very similarly at that point.
If you are affected so differently why not come up with your own personal score and base it on RPE, watts, time, CP, etc.