Don't have flammula but you are discovering what everyone discovers about clematis when research is done on them---that the plants have various listed growing heights based on the source of the information (Flower color, zone hardiness ratings, and when they bloom are additional info that you will find conflicting information on about clematis). To add more to the confusion, COTW lists the plant as getting 3 to 5 meters tall which translates into roughly 10 feet to almost 16.5 feet tall.

All you can do is take this information as a general guide and see how the plant does in your own garden. There are way too many factors that could affect the plant's eventual height such as soil fertility, length of growing season, age of plant, amount of sunlight/shade it gets, the microclimate the plant is grown in, etc.

If you don't prune group IIIs as they should be, that is hard in the winter/early spring, then the lower stems will get woody, have no leaves on them, and the flowers will be up higher on the newer wood that is produced each season. Type IIIs customarily bloom on new wood that is produced each growing season and not on previous year's wood.