Why does Apache show such drastic differences in memory usage when using mod_php?
The main difference is that different requests to a Drupal site will do different things.

Serving up a static image, CSS, or JS file has very little overhead and the PHP handler doesn't do anything.

Serving up a cached page invokes the PHP handler and Drupal bootstraps, but the overhead is small, compared to other requests.

Serving up a non-cached page will cause a full Drupal bootstrap, and all of the PHP necessary to render the URL will run.

Serving up an image derivative will invoke PHP, and there will be additional memory overhead.

Serving up a page with an empty Drupal cache will result in all of the overhead of rebuilding the memus, registry, etc, and can cause a pretty big memory swell.

So, I suspect you are seeing the result of processes serving up different types of requests.