Just like Song Yang comprehended 100% of the Sacred Law of the Soul after reaching the initial stage of the Seven Souls Scripture, he managed to comprehend 61% of the Imperial Law of Space after reaching the perfection stage of the Path of the Spatial Wave.
The Imperial Law refers to the laws understood by quasi-emperors, with 10% equivalent to a 1-Star Quasi-Emperor, 20% equivalent to a 2-Star Quasi-Emperor, 90% equivalent to a 9-Star Quasi-Emperor... and finally, 100% equivalent to the peak of the Quasi-Emperor realm.
As for why it is so different, it is clearly due to the classification of the techniques.
Even after comprehending the imperial technique; Path of the Spatial Wave to the perfection stage, Song Yang was only able to comprehend 61% of the Imperial Law of Space.
On the other hand, if Song Yang comprehended the Seven Souls Scripture to the perfection stage, probably, not even the integration of the Soul Law would be his limit!