There's an inconsistency but no ambiguity, only ignorance. Inflammable only ever means one thing regardless of how ridiculous english might be.

The historically correct term would be non-inflammable. The modern variant is non-flammable.

Similarly, inflammable is the historic term and flammable is the modern variant.

The confusion arises when people are exposed to the word flammable and then attempt to apply the usual rules to construct a word they've never actually used before.

This isn't the usual sort of inconsistency introduced by our fusing multiple incompatible languages. It's from the original Latin and I'm unclear what led to it. For example consider inflammable versus inhumane. It seems Latin itself used the prefix to mean different things - here on(fire) versus not(human). But confusingly it's ex to indicate location, despite ex also being the antonym of in. So ex equo means you are on horseback, not off it as I would have guessed.