You might wonder how the “blue” group could improve during week 1-8 by reducing their training load.
Potential explanations are many, successful taper, supercompensation, outliers and confounding factors. However, the authors address these possibilities with statistical analyses in the original paper:
EXP (me:”blue”) induced a very likely moderate increase in power output at 4 mmol·L-1 during the transition period compared with TRAD (me:”yellow”) (5.4±7.7% vs. -4.7±7.1% for EXP vs. TRAD, chances that the true between-group was higher/trivial/lower, 97/1/2 %)
Rønnestad et al, Eur J Appl Physiol 2014
In short: there is always a degree of uncertainty with any research findings. In this case, the chances of the results being “true” are deemed “very likely”.
Do not mistake the take-away for being that reducing your training load for 8 weeks will necessarily improve your capacity. Rather, the take-away is that regular high-intensity work every 7.-10. days seems to prevent a loss of endurance capacity.