Two people might utter the same excellent sentence, although their abilities differ. One has memorized it; the other can independently generate suitably eloquent statements on new subjects for different audiences.
Occasional success: a correct expression that the speaker cannot consistently reproduce and adapt.
Established faculty: a stable capacity to choose suitable meaning, arrangement, emphasis, brevity and explanation in varying situations.
The distinction does not make a capable speaker infallible. Occasional mistakes remain possible; the designation concerns a settled capacity rather than an assurance that every utterance will be perfect.