The "Dimension" tag of a summoned entity will always default to 0, no matter which dimension the entity was summoned into. The tag is not updated until the entity changes dimensions, such as travelling through a portal.

Steps to reproduce:

1. Go to the nether.
2. Spawn a cow using a spawn egg
3. Run

   /testfor @e[type=Cow] {Dimension:-1}

   • The result in the chat window will be "Found Cow"
4. Kill the cow
5. Summon a cow using /summon Cow
6. Run the same testfor command again
   • The result in chat will be "Cow did not match the required data structure"