Yeah exactly. That's why it's hard to 100% assume that just because the account was created on the same IP means it's from the same person. I know the fashion of a 'family PC' is outdated these days, but you can never rule out that possibility. But sadly, the moral of the story is, don't create more than one account from the same PC and/or IP address. Otherwise the family could share that one MTF account, but that's hardly a good idea
I read somewhere two accounts from the same address is something you should ask the mods about before registering the second one. Easier to get permission than forgiveness, I assume.