Different SE sites cover a wide range of topics. And the thought process in generating a question is VERY different from one to another. In programming related sites, I can come up with many question in a day just because I run into errors. Those can be very specific to my case, therefore been less likely that I'll find an answer already there. In software use related sites, a new software or version show up very often creating a whole new set of associated questions.
In the other hand, in Earths Sciences, the SE community I'm more active in. New fields of research don't appear very often, the thought process to come up with a question is much longer and questions don't tend to be that specific, so you are more likely to find the answer already somewhere on the internet.
As a consequence, Earths Sciences beta duplicates or triplicates graduation requirements when it comes to avid users or visits, but it does very bad when it comes to the average number of questions per day.
Given all the above: Does it really makes sense to have fix minimum requirements for all beta sites?
Also, similar differences can explain why some sites will have more answers per questions than others.