Notes, Notings, and Common Refrains

If you've always known the poem with a single set of illustrations, that would definitely influence your interpretation of the words.

And if that vague memory is correct, apparently, the illustrator saw the same inconsistency I noticed. But taken at face value:

"As I drew in my head, and was turning around
Down the chimney Saint Nicholas came with a bound"

Certainly implies an instantaneous moment.