Bryan states in the original HST instructions that it is LOAD that causes hypertrophy, and says do the minimum volume with progressive load to get hypertrophy, and you only need to do 1-2 sets. The general conensus on the boards (and Bryan has said a lot of stuff to support this view), that it is WORKLOAD that causes hypertrophy, ie. doing work/reps with a given load. If it was just load, and volume didn't matter, you could do 1 rep with your 1RM and grow like crazy. You still need to do work with that load.
Soo...a simple and very effective system that a lot of people on this board use (and I believe Bryan uses this as well) is volume constant HST, which means you set a target rep count, 15-30 per bodypart depending on your lifting experience, and then cluster your reps to make it to that rep with the load your using (always increasing), while staying away from failure. Ie. 1x15, 1.5x10, 3x5 is always 15 reps. 1x15, 2x10, 3x5 is close enough to that for it to be recommended to newbies, cause its simple. That's not to say that 1x15, 1x10, 1x5 won't give you growth, but people will find that 1x5 will not give good results, hence the misconception that "the 10s are for growth and the 5s are for strength". Doing enough work/reps with 5s weights will produce better hypertrophy than the 10s, because your workload is higher.