This is a very good question. There are several reasons. First, we all have differences in our bodies. In some women, there appears to be a very high tolerance for pain. These women are presenting completely dilated and say they have cramping and are ready to deliver.
Some women have very good control and can focus internally.
It does not mean that the pain is less intense than someone who is screaming. It may have to do with the way she focuses on that pain. I hope that answers your question. The actual mechanism, meaning the process in which women have nausea in pregnancy, is not well understood, and there are various manifestations. In a significant number of women, vomiting will be a natural part.