I'd guess it is on par with how some people in the Star Wars universe are "force sensitive" and, without training, never really become Jedi or Sith, or whatever else uses the force as a conduit for power. They're just people who have/had the potential, but never acted on it.
A magic sensitive individual may have varying degrees of access to the power they can feel. It may manifest as just a feeling when magic users are near to strange outbursts of energy in times of stress.
I don't think that is the case. There was a old story about a Jund warrior
Dragonsoul Knight where after journeying across all of New Alara he gained the ability to turn into dragon, though he wasn't actually a mage.