that is an altcoin, not a fork, the fork didn't happen theymos said so in his thread, bitcoin cash is an attempt to do a fork but they created an altcoin, but i don't know why they both share the same private key
Because it is forked Bitcoin with slightly different consensus rules. SegWit Bitcoin has slightly different consensus rules as well, but at the end it is what most exchanges/companies/people agree what Bitcoin consensus rules are. Thats why Bitcoin Cash is considered altcoin by most, and only minority view it as continuity of Bitcoin.