Why process.memoryUsage() doesn't output the consumed memory of the node process

In node.js application, I use the code 'console.log(process.memoryUsage());' to log the memory usage. The output is as below.

{ rss: 13664256, heapTotal: 6131200, heapUsed: 3396912 }

When I use the process monitor to check the memory usage of the node process. The memory size is 14.5M.

I have a couple of questions.

1. What do the rss, heapTotal and heapUsed mean?
2. Why process.memoryUsage() output a memory usage value which is same as that shown in the process monitor?

====UPDATE 6/19/2013===

My OS is Mac OS X snow leopard.

Regards,

Jeffrey