negated negation

A prefix which should, but does not, negate the word to which it is attached. While 'mobility' and 'immobility' are opposites, as are 'effective' and 'ineffective', the words 'passivity' and 'impassivity' mean the same thing, as do the words 'flammable' and 'inflammable'.

Tired of being described as passive, he decided to be more impassive. He'd be a man of action, or maybe even inaction. What difference could a negated negation make?