This is part of the behavior files in the Havok Behavior Tree used by Skyrim. They implemented some blending between walking and running animations. To remove this walk-run blending, you would need to modify Skyrim's behavior files. In my opinion, the developers blended these animations as a shortcut to save effort. To be honest, that feels confusing. It doesn't seem like the work of a mature, industrialized company. You can modify behavior files using the Skyrim Behavior Tool available on GitHub.
