It wasn't that Masters did not exert more pressure than Seniors. They most certainly did.
However, today, Rui was able to distinguish the origin of the aura.
What he meant by that was the fact that the fear that Martial Seniors were able to inspire from Martial Squires was something that came from the power of the Martial Body. Rui could sense that the origin of the fear that they inspired from lesser beings was the power of the Martial Body and the Martial Heart buried deep within it.
Of course, their Martial Paths, and Art too inspired a great amount of fear, but that was something that was true even for Martial Squires, what set them apart from Martial Squires was the latent sense of pressure that the presence of the Martial Heart exerted. That was what made the auras of Martial Seniors distinguished from Martial Squires.
However, Rui was unable to identify what it was that set apart Martial Masters from Martial Seniors.