Why am I getting a response from version 1131 when I passed lower version in the header?

The version number (<Version>xxx</Version>) returned in the API response represents the latest WSDL used to process the request, and it should be used for troubleshooting "CustomCode" values that appear in the response.

You need to specify the version number in both the SOAP header and the Request Payload to let eBay know that your application complies with the compatibility level that is in effect for that version, and eBay will uses the rules that you specified and return the functionality and tags based on the schema version specified in your request in most cases.