another good suggestion is to stay away from scam website such as bitcoin.com that would dump an altcoin on you and call it bitcoin. there has been many cases over the past 2 years where a newbie went to this site and bought what they thought were bitcoin but ended up with a shitcoin called bitcoincash.

there is nothing to be surprised about. when you copy something you are copying everything including address formats. and that's just the tip of the iceberg because blockchair only has 2 of these altcoins otherwise you would have seen about 50 different addresses from 50 different copies of bitcoin.