I recently came upon a source that sheds light on that conundrum.
In his autograph will dated March 1, 1673 (AN, MC, XLIV, 47), a painter named Isaac Moillon alluded to his "ordinary" handwriting: "J'ay escrit le mien testament dans ma propre main & de mon escriture ordinaire, et j'ay signé ... Moillon." This sentence suggests that the painter had mastered two distinct scripts; that he routinely chose one or the other according to the circumstances; and that he feared the will might be attacked because the handwriting did not correspond to the writing with which an heir was familiar. For his will, Moillon opted for his "ordinary" every-day hand. Like Moillon, Charpentier had an "ordinary," day-to-day signature and script (we have become familiar with it not only in receipts, registers and notarial acts, but also in the lyrics preserved in the Mélanges); and he had an archaic calligraphed signature and script that he apparently reserved for momentous events such as the guardianship of his niece and nephews and of which we, thus far, possess only one example.
See also Sylvie Béguin's "Pour Isaac Moillon," in Mélanges en hommage à Pierre Rosenberg (Paris: Réunion des Musées nationaux, 2001), p. 72.