Aven Crest nobility started as any other region- the lines of nobles prospered and philandered, deeming themselves better than others due to their bloodline. This attitude is still seen in the five original noble houses, though in few others.
Now, anyone fortunate enough to purchase an estate in the nobility district is considered nobility. There is very little real power in the title, other than bragging rights. Power comes from information and money in Aven’s Crest, and that, too, is where the true power of the nobles comes into play. Not their bloodline.