As a hub matures its score goes up slightly. Also depending on the individual words that make up the title and the amount of traffic searches that each individual word receives on the site, then that also contributes to the rise in traffic.
To validate this you can try to write a hub with a simple title and then change the tittle to a more complex set of words.
For example; Mushroom Recipes uses words which are too common and not in a niche. However : Shitakee Mushroom Recipes New York Style, has some words which are in a niche (Shitakee and New York) and therefore would receive an initial score of a higher value.