nic, I'm pretty sure Acute means: sudden (as in its onset) and severe (as in degree). Chronic as opposed to acute, speaks of the duration of time since PTSD was diagnose(able). Another words, a traumatic experience sustained today in an individual without PTSD, when left untreated could not only develop into PTSD months later and be diagnosable then, but would not be of a chronic nature, ...yet. Not until one lives with it for a long-standing period of time and, in my experience, finds that PTSD (it's imbalance) hasn't up and gone anywhere; then it could be classified as chronic.
So I'd figure that yes, everyone that goes on to live with PTSD for a long duration of time would then have chronic PTSD. But, it doesn't necessary mean that chronic PTSD spells any greater disaster, it's possible that with it's long-standing duration it may have become easier in some respects for an individual over such time to have accepted and processed much, and to manage it then for another who is still suffering in it's acute stages.