Another explaination behind the answer...

With regex's and meta characters, it depends on whether or not you want to join the meta character with the pattern as a single atom.

If the meta character is placed in the parenthesis with a pattern, then it'll be included in as part of the backreference variable.

In your case with using the meta character \b, the "word boundary", it is not something that represents (a) character(s), therefore it wouldn't show up in the backreference variable.