Asian virginsbower is a vine native to southern China and grown in U.S. gardens. The wild form has six white flower parts that look like petals, while the 'Sieboldiana' garden form has a purple center.
Patens clematis (Clematis patens) often has eight showy sepals and lacks the pair of leaflike on the flower stalk. Asian virginsbower usually has six sepals and a pair of bracts. Some patens flowers have fewer sepals, so check the stalk too.
Why they get mixed upBoth are grown for broad, open flowers.