Varying (non-deterministic) result order with unified search

When I search repeatedly, my results are ordered differently. Not every consecutive search changes the order, but when I recorded the top ten results of five searches, there were three different orders (with two repeated twice).

I've reduced my code to just 

var search = SearchClient.Instance
                .UnifiedSearchFor(query)
                .Take(10)
                .GetResult();

Any tips on how I might go about debugging this? I have tried manually running the indexing job.

Edit: Grammar.