I'm really not sure which explanation to go with for Brock.
A. Brock was this elite fighter, beasting through competition...until he got sick. He came back to fight weak as hell, was able to somehow withstand power punches from Carwin, but showed signs of weakness for the first time. Then in his fight with Cain, Brock was too anxious when striking, which lead to him falling victim to Cain's precise blows. Cain won, but only because Brock is a much worse fighter than he was before getting sick.
B. Brock was physically gifted with strength, power, and size over most (if not all) his opponents. He came from a great wrestling background, but ultimately his physical strength is what edged him over ALL his opponents. As his doctors (not just random people online) stated, Brock had been fighting at 80% of his full strength prior succumbing to his sickness. He recovered, and was cleared to fight. His doctors (again, not random people online) stated that now he was actually at 100% strength for the first time in his MMA career. He fought a really powerful striker that, by many people's assessments, had Brock beaten, but the ref didn't call it. Went to a 2nd round where a gased Carwin was submitted by Brock. Then in Brock's next fight another powerful hitter, with much better cardio, again tested Brock's chin (which, until Carwin, had never been tested
on that level before). Brock cowered and fled under the strikes until the ref, THIS TIME, called the match the right way.
I'm not sure which explaination to go with.