E.g.: if you create abcdef@gmail.com, then a.bcdef@gmail.com , ab.cdef@gmail.com and so on become unavailable and are directed to your main account.

Click on "Your address is similar but has more or fewer dots (.) or different capitalization." to expand the explanation.

We had to add a link to this support document to the security@google.com autoresponder because people misunderstand this behaviour so often (They think they're getting someone elses mail, but someone just sent it to the wrong address).