How comes one minute O' senseis body feels one way and the next minute - at will - it's all soft ?
How come there were "muscles bulging al over his body"
Tensing or flexing makes your muscles makes them harder. Relaxing them makes them softer. Body type size has a lot to do with it. O'sensei was pretty short. If he was over 6 feet tall, it might have been different.
Training all your life is also a major factor. I used to work for a Kung-Fu/Karate shop and the owner was super solid and up there in age.
I saw him balance on one leg and squat down to the ground with the other leg fully extended, then get back up without using his hands or arms.
A lifetime of training and body conditioning is why he can do that.