Staging environments typically exist with integrations with other systems that have references to auto-number fields
Data deployment re-evaluates auto-number fields instead of keeping the values of the source.

If data deployment jobs could automate the following it would probably work
1. changing the type of the auto-number field to text in the target org
2. potentially removing apex references to the field
3. deploying the data
4. rolling back metadata modifications in steps 1 and 2