Poida's onto something here.
While it's not all that clear why this is the case, the more frequently you go through a movement, the less sore you get from it. I've generally observed that those who train a muscle once a week are always sore, and for many (far from all, but a large enough proportion to not be total outliers), this is true regardless of how hard they train in a given. That was certainly the case when I moved from full-body to splits for the first time: constant soreness. I had been taught that if you're still sore, then you're not ready to train again, so I progressively kept on spreading my bodyparts further apart while training, and kept getting sorer for longer. Some experience what I've experienced in that regard, others stop getting sore from a certain workout plan within a couple weeks regardless of frequency.
Right up the other end of the spectrum, I've found for myself that if I do fullbody 3x/wk after a layoff, I'll be brutally sore for the first week, then, after the first workout of the second, I'll barely feel any soreness at all, and this will continue to be the case for the life of the program. Right now I'm being completely stupid and squatting 6 days in a row (which I do NOT recommend outside of specific circumstances, and inside those specific circumstances, I still don't recommend it so much as acknowledge it as a possibility). I had some vague soreness this morning, on Day 3, but it was actually from cramping my hamstring outside of actual squats. The squats themselves are leaving me totally unsore at this frequency, whereas if I had done Monday's workout and then not trained my legs for the last 2 days, I'd probably still be a bit sore.
EDIT: In saying all this, and bringing it all back to the original post, what I'm doing right now is not a bodybuilding plan. I can't remember the details, but I have seen some evidence that if you take a certain total weekly volume of training for a muscle group and fit that entire volume into 1 session, the hypertrophic results will likely be greater than if you spread that volume out throughout the week. So, say I do 3x10 bench press on Mon/Wed/Fri, or I do 9x10 bench press on Mon, I'd likely build more chest, shoulder and triceps muscle mass in the latter scenario than the former.