Markets in some groups are excluded from the search results. (Such as the Destiny.gg group.) Supposedly this is because some people don't want to see them, but this doesn't make a lot of sense because the whole point of a search function is to allow people to choose what they want to see.
This "feature" is not communicated anywhere on the search page, nor indeed anywhere on the site at all as far as I know, and has led to confusion and bad bets because people made the mistake of trusting that the search results were actually accurate.
Resolves to NO if I'm aware of any public market that I can't get to show up in the normal search. Otherwise it resolves to YES.
For anyone looking for a way to search all public markets, this is now possible with a workaround, see here.