This is an interesting topic and one I've thought about quite a bit because of my personal situation.
I grew up with only my mother and my younger sister in the house and neither are girly girls in the least. My mom hates wearing makeup, messing with her hair and loathes anything but jeans and a t-shirt, she doesn't mind mowing the lawn or killing bugs; my sister is pretty much the same way and neither have a problem hanging out with guys and giving them crap. I've never really thought either of them to be tomboys though, but maybe my perspective is different being that's all I was exposed to growing up.
It also makes it even stranger that I've always been much more "girly" than either of them since I was a child. I like playing with my hair, doing makeup, and enjoy a nice skirt as much as I do jeans and a cami. I'm also terrified of bugs and hate to be dirty, not because "girls are supposed to be that way", it's always just how I've been. I've always thought it to be quite interesting that these have always been my traits even though I was never exposed to any of it growing up. Nature over nurture for sure in my case.
I would consider myself a bit of a tomboy though, I will always love cars and mechanical things, but I'm not sure how much of that was influenced by growing up as a guy or has always been present in me. I have clear memories of reading popular mechanics magazine at my grandmothers house as a small child and absolutely loving it.
I dunno, it's interesting where we all end up in this "spectrum".
~Sara (who currently has grease under her fingernails but nicely pedicured and painted toenails)