@JoanD. Boy, that's a tough one, Joan. Sometimes, simply because these plants don't get the same amount of sun, were not fertilized/watered as diligently, or are growing in poorer soil, different stands of trees will behave differently. I don't know where you are located (it would be great if you went into your "Profile" and listed that information so that it shows every time you post) but perhaps there are college-trained horticulturalists, perhaps working for landscape companies, that can help ID those new plants. Your County Extension Service might be able to help or can refer you to someone who can.
By the way, I have a stand of Green Giant Arborvitae that have also grown really fast. Some of that stand of trees gets about 2/3 the direct sun of the others, and they are all about 25% smaller. It is simply the shade that causes the difference, since all other growing conditions are the same.
