User blog comment:Rappy 4187/Technical Update: February 4, 2014/@comment-1361074-20140205200320/@comment-957747-20140206043237

DPL, by design, isn't meant to deliver random results every page view. Yes, randomcount is something that DPL does, but DPL is meant to cache the results for performance reasons. Once the page has cached, that same result will be shown to others so that DPL doesn't have to re-run the query. DPL will never give you truely random selections without user input.

I would suggest looking in to the DPLcache parameters if you feel the current cache time is not meeting your expectations or not rely on DPL for randomness. You can find the cache parameters here.