It seems there are several disjunctions between wgContentNamespaces and wgNamespacesToBeSearchedDefault. The latter is what is reflected as "Content" in the Special:Search page.
This was noticed because "Risotto" doesn't return any results on en.wikibooks, even though there is the page Cookbook:Risotto. I have created a diff list between the two variables using P5740
[] Not searching 102 for +aswikisource
[] Not searching [100] for +bnwikisource
[] Not searching 102 for +enwikibooks: Cookbook namespace
[] Not searching 110 for +enwikibooks: WikiJunior namespace
[] Not searching [102, 110] for +fawikibooks
[] Not searching 100 for +hewikisource
[] Not searching 106 for +hewikisource
[] Not searching 110 for +hewikisource
[] Not searching [102] for +hrwiki
[] Not searching [104] for +ltwikipedia
[] Not searching NS_HELP for +metawiki
[] Not searching [102] for +srwikibooks
[] Not searching [100] for +srwikisource
[] Not searching [100, 110] for +trwikibooks
[] Not searching NS_HELP for +wikitech
Some of these might be desirable, but likely most not.