It is easy to learn asanas in a yoga session. Our body gets more flexibility after a few months of practice and postures get easier with time but physical lightness, strength and healthy neuro-muscular system could be achieved with any other physical training and is only a small path in yoga discipline.
There are all different types of themes that yoga instructors use. Some are for the type of students in the class, i.e. gentle yoga. Others are structured around a specific body part or type of pose, like backbends.