I think the short answer to your question is neural adaptation. When you get in the groove of a particular set/rep pattern, your body adapts to it. When you add a bunch of reps from what you’re used to, it can be very tough at first. But your body can adapt to the new rep scheme fairly quickly.
This was your first time doing your brother’s workout. If you gave it a few more shots, you would make a whole lot of progress quickly, and then probably stagnate a bit, just because your body is getting used to new patterns.
I’ve had this happen to me when I switched from Thib’s HP mass program to a Dan John 2 day a week program. It was pretty much a complete 180 from what I was doing (I made the switch because I had to work 70 hour weeks for a short period of time). I was shitty at the higher rep stuff to start, but within a few weeks it felt natural and I could use weights that made more sense to me for my strength levels. Hope this makes sense!