The confusion stems from your thinking \b matches spaces (probably because “b” suggests “blank”).
\b matches the empty string at the beginning or end of a word. \B matches the empty string not at the beginning or end of a word. The key here is that “-” is not a part of a word. So <left>-<right> matches \b-\b because there are word boundaries on either side of the -. On the other hand for <left> - <right> (note the spaces), there are not word boundaries on either side of the dash. The word boundaries are one space further left and right.
On the other hand, when searching for \bcat\b word boundaries behave more intuitively, and it matches ” cat ” as expected.
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