What's the exact difference in the 'pi' sound between 'happiness' /ˈhæp.i.nəs/ and epicentre /ˈep.ɪ.sen.tər/ that prompts the Cambridge Dictionary compilers to use a diffrerent vowel code for each case? More generally, what's the exact difference between the /ɪ/ sound and the /i/ sound, is it just in the duration (in some contexts?)? Is it just a historical variation that's lingereing? Or is there something more to it?
Edit: After posting this question, I hit these two blog posts that are discussing the issue in detail:
One takeaway is that the /i/ sound was introduced to denote possible alternative pronunciations /ɪ/ and /i:/.
After reading the two posts, I still have questions:
What's the point of using the /i/ in the middle of a word, I'm not aware that there are alternative pronunciations for happiness?!
Is there still accents around that pronounce happy as /ˈhæp.i:/? And why is it not just considered allophonic variation anyway? When I listen to the standard dictionary pronunciations they seem to be somewhere between an /ɪ/ and /i:/ in duration, but in real life I hear native speakers normally pronounce it as /ˈhæp.ɪ/.