This is actually true for many forms of physical training, including not just strength training but also endurance training (running, etc.) Plateaus and injuries aside, the basic principle of training is overload followed by recovery, and it's not uncommon during the overload phase to have high levels of fatigue and minimal improvement if not regression.

This is something a lot of casual athletes don't notice because there is a very steep development curve for the untrained, so you just getting better very quickly. Once you have been training for years, gains come much slower.