They only average out if they're uncorrelated. I'd say it's pretty likely that high-budget, expensive games are more likely to be released with DRM than low-budget indie ones. You could try randomizing on budget, dev size, etc..., but there's really so many random variables to account for in game development (expected sales? platforms released on?) that you'd need a ton of data before you really got something that's statistically significant.

Although I'm no statistician either, so whatever.