For instance, I have one son who could sleep 14 hours a night and the other one, who is almost one year old, likes to sleep only about 7 hours a night. Children learn to sit, crawl and walk at different ages and in different ways, too. These children developed speech sounds differently. We also know that frequently these children had no direct need for speech therapy too. With these questions aside, it has been shown that children do have similar patterns of development, generally speaking, as far as speech sound aquisition goes.
Children learn to do eassier tasks before more difficult ones.
The same holds true for speech sound development. Children are more apt to produce sounds that are easy to see, such as sounds made with the lips than the more difficult ones that have subtle features and tongue movements like an R sound. This has been found to be true universally. Fellow therapists and researchers have acknowledged these facts in other languages also. I have found through the years in my profession that parents and other caregivers frequently do not understand what sounds are appropriate for children to begin using at different ages.