Sex plays a role
Look, I'm a woman, and I'm loathe to admit it, but where muscle mass is concerned, it's very often the case that men can build more of it, and quicker, than women. This certainly doesn't mean women can't build muscle, increase strength, or develop a svelte physique. Quite the contrary.
What it does mean is that if you're a woman hitting the gym with your guy friends, the dudes are probably going to build muscle faster than you.
Dr. Kendall explains that some of the discrepancy is due to gender differences in the makeup and composition of muscle fibers, "Women have a significantly higher percentage of type I muscle fibers in their quadriceps compared to men. Furthermore, studies have shown greater size of type II fibers in men compared to women, and a greater ratio of type II to I fibers." This is significant because type II fibers are associated with high force and power production, and their ability to experience more significant growth.