If a GraphQL query returns two arrays, the second array will have an id field added to each object in it, apparently set based on the current UNIX time.
The id is added by JSON data parser, not the proxy - and it appears only after parsing (please, check the console): Code Snippet
While GraphQL server is just a data source, Webix widgets add the unique id for each parsed record (that do not have ids of their own) to maintain data coherence and provide support for all the client-side CRUD operations and filtering/sorting.