I expanded the post a bit.
I explained some of the underpinnings of SDT in the past, as an example. One principle, whether right or wrong (I think right!) is that adding volume to an existing number of reps and sets cannot be seen to correspond to some certain increase in your maximal ability. Likewise, and by extension, adding an arbitrary workload to an existing number of reps and sets will not automatically add an appreciable amount to your maximal ability on an ONGOING basis. That is, week in and week out.
No principle is a golden rule. It's just an underpinning (when it comes to strength training). So, for example, it's a given that if you do 200 x 4 x 6 one day and then next time do 215 x 4 x 6 (i know, as if, but it can happen) then you know at least that you can lift 215 pretty damned well. But you expect a certain, corresponding increase in your 1RM. Well, you are not guaranteed one, because of the interplay of fitness, fatigue, and fluctuations in performance, right? So what you do, in terms of workload, over two workouts, does not automatically mean you are set to get a new PR. But, if those two workouts are a further continuation of a buildup that started with a lower volume and much lower weight, and at some point you return to that starting volume and add an appreciable amount of weight, over what you have done before, to it..you are guaranteed to have increased your maximal ability by some significant amount. Of course, what is significant becomes lower and lower as you advance in your training.
All of that is one underpinning, or "principle" behind SDT. The specific principle being that progression can only be measured by relating it to set BENCHMARKS that are established at some point in time. The benchmark in SDT would be the BV and the progression is going back, after adding workload over time with reps, sets, or weight and adding weight to the BV. This is easy to see when people are actually doing maximal lifts. They hold their 1RM or PR as a benchmark (although that is another story) and they progress from there. But they do not extend this to other intensities all the time.
And you have to see that the principle is very important. SDT is floating around and other people are writing their "own" versions of it but being completely unaware of this principle. So you're adding stuff to existing sets and reps, so what? Hardly rocket science.
Now, what I said about fatigue, fitness, and fluctuations in performance I'll qualify a bit because otherwise that will just sound like vague jargon. You know I've written at length about fluctuations in performance so I don't need to go down that road again. But at how people are always harping about recovery. Well, lets say you pull off adding 15 lbs to that 4 x 6. Are you more fit, or are you more 'recovered'? In other words, have you gained new ability that could be seen as meaning new maximal ability, or, have you simply uncovered existing ability? This is why we don't measure progress, in terms of maximal strength, over one, or even two workouts and why something like SDT (and the Single Scene) is meant to cause progress over a cycle of workouts, SDT being open-ended and SS being set.