Hey
In the basic HST article, it says "1st week [of 10's and 5's] do 2 sets, then on the 2nd week do 1 set".
Why?
Simple. As the weights get heavier, you really can't expect yourself to be able to do the same amount of volume as a lighter weight.
Understand that when Bryan set up the "vanilla" HST routine, it was a compromise of a lot of factors as I understand - so that beginners won't get lost, do too much, or feel they failed to accomplish something. In the same way, even "advanced" bodybuilders would still get benefits from following it.
So as the weights get heavier in a specific rep scheme, it was advised to do just one set - simply reflecting the fact that you are dealing with a heavier weight now and probably can not do as much volume as before. Simple rule, really:
as the load increases, the volume decreases. Not because we want that to happen, but simply because it does.
Now, some people choose to do more. It can be for a variety of reasons - they've been training for so long they need more to stimulate hypertrophy, or they've been used to an unusually high number of reps/sets thanks to HIT or whatever, or they just "feel" they have to do it.
Is that bad? Not necessarily.
Again, we go back to the volume rule:
Do as much as you can. This is of course always in the context of doing as much as you can without sacrificing your ability to train more frequently - which of course means closely monitoring your fatigue levels, avoiding training to failure regularly, etc.
This is actually pretty hard to apply. Some would tend to overestimate what they can do, and as a result they end up getting fried, then complain "HST is no good!". Really now.
That's why Bryan outlined in the first place to do 2 sets in the 1st week, then 1 in the 2nd week - as you approach your rep max, it obviously becomes the case you won't be able to do as much as you did in the previous week. By the time you are in your actual 10RM/5RM, you most probably can only get one good set out, and most probably that's really all you need to do anyway.
Regards,
-JV