Why did the scales of obedience at first tilt in Aura's favor?

All Freiren is doing is merely 'hiding' her mana, right? Meaning even though its hidden and those around her see a false representation of what it is, the mana itself as a whole is still there.

Meaning when Aura used the Scales of Obedience against Frieren, shouldn't the scales have immediately tilted in Freiren's favor? I don't see how she could 'fool' an item like that. Even with her hiding her mana, the 'weight' of it should have still been the same.

Thus, Aura would have been completely bewildered as to why the scales favored Frieren, then deduce that she was partaking in the 'shameful' act of hiding her mana. Thus prompting Frieren to show us the big reveal of what her actual mana was.