Why does a Q&A search for "circled r" show only reirab’s answer about private airports and not PMoubed’s question about the Communications section in the Chart Supplement? Both contain the phrase in question.
Short answer: Because the site's built-in search sucks.
Longer answer: Stack Exchange sites are Q&A platforms, not search engines. The built-in search is kind of a best-effort thing: Sometimes it's good, other times it misses things or pulls up irrelevant pages.
Use Google if comprehensive results are important.