The NBA playoffs are going on, so I would expect an ebb and flow of news stories, Facebook posts, tweets, etc. which could easily vary by that amount. To illustrate via an admittedly more extreme example: I'm certain that the number of results for a search on "Game of Thrones" last night would've been much greater than for the same search the night before...
Another possibility might be a fluctuation in the percentage of Google's millions of servers that are online at the moment you're searching.
