Since there are 180million of them, presumably they were generated with a computer. Then, it seems that they probably fit into a finite number of patterns. If these could be determined, wouldn't this be a rather easy to crack captcha system?

Most captchas depend on the difficulty of the reverse transform applied to an image, especially when you don't know what the transform is. Here, the forms seem pretty regular, and the "transform" of inserting words is discrete rather than continuous, so a bit easier to reverse.