5.21.8 Limitations of the variables
By putting variables in documents, the documents become simple templates which generate different content depending on the context in which they are displayed. If the user sees a word in a document which results from a variable, and the user searches for that word using the fulltext search, then the document will not be part of the search results since that word doesn't really occur in the document.
Similarly, searches on document names search on the not-variable-resolved document names.
The variable names are part of the fulltext index, so you can search where variables are used.
Variable substitution in document names is currently not performed everywhere in the Wiki. The basic document display (documents, navigation tree, queries embedded in documents) are processed, but in other locations such as the document browser used in the editor etc. the document names are are unprocessed, since authors are probably more interested to see the raw document name.