Why are the numbers on segmentation different than smart list if they both have the same criteria?

Hi. 

I can't provide snapshots due to sensitive information, but can someone explain to me why my segmentation (which is used for a dynamic email) has a smaller number of people than the smart list? I actually created the smart list first. I then realized that segmentation was better for the type of email I am creating. So, I used the same criteria that is on my smart list to build the segmentation. So, why aren't the numbers the same?

Thanks!