Let me try again:
Suppose I "own" black.hole@gmail.com. Obviously I will receive emails addressed to black.hole@gmail.com, and all my outgoing mail will have black.hole@gmail.com in the "From" field, but I will also receive any emails addressed to any email address with any combination of dots (or none) inserted into the string "blackhole", appended by "@gmail.com". Gmail regards them as equivalent.
eg: blackhole@gmail.com; black..hole@gmail.com; b.l.a.c.k.h.o.l.e@gmail.com
I don't know whether this also applies to @googlemail.com, I presume it does.
Now suppose I have an eBay account linked to black.hole@gmail.com. Somebody else (or me) could register another, different, eBay account to black.ho.le@gmail.com, and eBay would see that as a separate email address and accept it, but I would receive the email traffic resulting.
Generally, situations like that are now more tightly controlled, and a confirmation email is sent to the target address to ensure the applicant has control of that email address, but it used to be common that there was no check - particularly if an existing user changed the address in their user profile rather than setting up a new account, and there could still be a lot of less-sophisticated websites that are not carrying out those tests.
There was an instance when I was assured it was a genuine mistake, but I find that difficult to believe unless black.hole is only one letter different from another bona fide address. The obvious reason is fraud - like Uber, obtaining goods or services with the bill going to somebody else. It can then be difficult to resolve, first to convince the supplier you are the injured party (because your email address is similar but not identical to the registered address) and then that you are not the account holder and not liable.