That figure is a mid-range one, and the spread across detraining studies is wide. What moves it most is how trained you were beforehand. Short-term losses look mostly central: plasma volume and stroke volume drop faster than anything in the muscle, and they come back just as quickly once you train again. Strength holds longer than endurance, so a break reads as a fitness problem when part of it is a blood-volume problem. Resting heart rate tracks that change better than how the legs feel.