"DuckDuckGo gets its results from over one hundred sources, including DuckDuckBot (our own crawler), crowd-sourced sites (like Wikipedia, which are stored in our own index), Yahoo! (through BOSS), WolframAlpha, Bing, and Yandex." [1]
Because it varies widely :).
It is entirely based on your query -- whichever source is detected as best for that type of query (i.e. Yandex is better at handling symbols, Bing's advanced syntax is better [mostly]), and of course it changes over time as rules are tweaked and added.