Thank you for your reply Stephen,
I guess I am confused. My understanding is that em size is related to the containing element, which in this case is Body. Any subsequent containing elements with width assigned would affect the em size within those elements. If the Body element has no assigned width or size and font-size is set to 100% then 1em = 16pixels? I think I have that configuration in my test file.
em is used for the reason you state, if someone increases the browser font size or zoom factor then all the elements (hopefully <g>) re-size all proportionately to the increased font size?
I still can't figure out why my em is based on 20. I'm testing this with Chrome.