Your problem is that you assume that TSS is correct. My data shows TSS is not correct.
When I do a FTP test, I hold the same power (+/- 10w) from 30 seconds in the test until the finish. My heart rate rises quickly to 75% of max. Then my heart rate increase slows to a leisurely pace. Reaching max within a couple minutes of the end. At that point it is very difficult to maintain my power out.
I think you are missing the point... TSS is not a measurement of anything, any more term "hard" a measurement of anything. It is an attempt to quantify "hard" or "easy" with workout quantifiers that are not individual specific. My easy might be your hard or vice versa. TSS does nothing more than attempt to categorize workouts with a scale that is universal. If you have your FTP set correctly, your 80 TSS ride will compare relatively well to my 80 TSS ride in terms of the training adaptations made as a result of that effort.
Now, FTP is defined as the result of a specific test protocol which results in a measured result known as FTP. TSS is an arbitrary term defined as 100 TSS = a 1 hour effort at your individual FTP power. Thus, if your FTP is correctly determined, it is impossible to have a 1 hour ride accumulate 120 TSS points since by definition, FTP is the max power you can sustain for 1 hour. 120 points means you rode above your previously determined FTP for 1 hour, thus, that would be your new FTP, and you would have accumulated 100 TSS, not 120.
I am not an acolyte of Dr Coggin or anyone else, I am however an engineer with a reasonably good understanding of science, power, work and related concepts.