[dns-operations] question about a host

[Eliza]

Hi,

I found 1.1.1.1's PTR has a interesting host:

$ dig -x 1.1.1.1 +short
one.one.one.one.

Then I query this host:

$ dig one.one.one.one. +short
1.1.1.1

It can be resolved. why this trick works?
  but two.two.two.two gets wrong...

thanks.