I was told long ago that it was the "Dynamic" half of the processes - the "max" you set is always there, and an additional number equal to that number are spawned as necessary.
But in my experience they're almost always spawned, they just only stay for about ~6 seconds. At least 2 "temporary" ones are spawned about 60% of the time, checking at about 1 second intervals for me. My server load is at .4, and there's not any waiting in the queue when I check on the website - the 4 "base" processes should be enough to handle it really...
This is annoying because even though it's copying-on-writing the memory from the parent process, rails still does a bunch of junk unique each time and so the memory gets dirtied pretty quickily. If you're at all short on memory this means thrashing and really low performance.
I'd thought it was more fixed in the past, but perhaps not. Perhaps the most recent patch added problems back again too though, it's hard to tell...