Did they use a curing compound after they finished it? If so it will sometimes puddle and cause the cement surface to look different where it puddled. You say that it is hot and dry where you live. I doubt that they used any additives when pouring it. The most common ones that are used speed up the setting processing, not something that you want to do on a warm day. If there was more than one cement truck load that can also cause the cement to be a different color just like any mixed product. Usaully over time it will bleach out to be the same color.