Environment variables and where they go

I noticed something really odd about environment variables in PHP under certain conditions. At my job, the development box is using IIS with PHP as an ISAPI module. One server that we are using for production is using Apache with PHP as an Apache module. Thing I noticed is that on the Apache box, environment variables are in the $_SERVER array and the $_ENV array is empty, while on the IIS box, the environment is in $_ENV and not in $_SERVER. Is it possible that there's a setting I may have missed to control where environment variables are stored? Or could it be a bug with PHP and or Apache, or just something on how the two work that I don't see?