I don't know if its because these areas are where muscle connects to bone, but as a lee and ler, I can tell you the areas you describe seem to be pretty common to those who are ticklish - I know they are on me. I myself am a fan of light tickling and I've tickled a few ladies over the years who were not at all tickish to the light touch, but I could get a response in those areas you mentioned with the harder tickling. I currently have a playmate who is actually less ticklish in her ribcage to the harder tickles but she really responds to the light tickles there. We recently discovered she's even ticklish over most of her back - my light "spider tickles" send her into orbit.