In PCRE, following a backreference by _any_ number seems to turn it into an octal value. Why is this?

Eg.
`(a)\1` <-- Backreference
`(a)\17` <-- Octal '17'
`(a)\19` <-- Octal 1, then literal 9
`(a)\1a` <-- Backreference, then 'a'