[davmre]

NIPS gets a ton of submissions, so the law of large numbers governs pretty strongly. Imagine that each paper submitted is independently either good or bad, with 22.5% probability of being good. With 1660 submissions, the total number of good papers follows a Binomial(1660, 0.225) distribution, which has mean 374 and standard deviation 17. Under this model, the fraction of good papers would be somewhere in the range 20.5-24.5% (corresponding to a two-standard-deviation window around the mean) in 95% of reviewing cycles. So even though the quality of the individual papers is totally random, the randomness mostly "cancels out" and the overall number of good submissions is relatively constant.

Of course this is assuming an objective standard for what constitutes a "good" paper. As others have pointed out, the only really meaningful standard is "how does this paper compare to other work being done in this field"? So it's also reasonable to think of NIPS's goal as just trying to present the best papers that were written in any given year, not as bestowing a strictly-defined stamp of objective quality.