Spine optimizes the update process by only calculating the bones and constraints that are part of the currently active skin. If your character is using the Normal skin with 4 bones, Spine will only update those 4 bones. The bones and constraints that are part of the Exploded skin with 25 bones will not be calculated unless that skin is active. This optimization helps in reducing unnecessary calculations and improving performance.

@fshakhverdiyev Spinebots answer is correct but might not be clear enough:
Note the skin-bones are only processed when the respective skin is active:
http://en.esotericsoftware.com/spine-skins#Skin-bones
This is not true for "normal" bones. If you don't associate bones with skins and e.g. just have empty slots, the runtimes still update your bones as you might want to query bone location for e.g. letting particle effects or a held weapon follow the bone position and rotation.