Source weights are computed by this demo, not returned by Jev. Jev only returns the answer and its probabilities.
For each selected source, the demo runs Jev once more with that source removed from state.search_results.
The weight is the winning probability with all sources minus the winning probability without that one, in points. Positive means the source supports the answer, negative means it pulls against it.
The Weights tab in the Request drawer shows the raw array, one number per source in evidence order.