User blog comment:DE-Note/What Determines the Strength of One's Aura?/@comment-10380701-20150529003752/@comment-10380701-20150529104942

Assuming genetics is at work, the possibility of having higher/lower aura levels may be also determined by how high/low the aura levels of your parents. But there's some luck factor thrown in because fuck the status quo of genetics.

I'm mostly basing the aura level theory to the magic circuit all the way from the Nasuverse.

Because it just so happens that one individual hailing from a magus family known for their below-average magic circuits gets born having something along the lines of twenty. Her parents had 0.