CustomBrowse < itemformatdata> question

Could anyone please point me in the right direction? I am playing around with my first custom browse menu, just taking baby steps displaying albums in alphabetic order. Here's the code, stripped down from Erland's built-in plugin:

<?xml version="1.0" encoding="utf-8"?>
<custombrowse>
<menu>

<menuname>Albums by title</menuname>
<menuorder>17</menuorder>
<menu>
<id>album</id>
<menuname>Songs</menuname>
<itemtype>album</itemtype>
<itemformat>album</itemformat>
<menutype>sql</menutype>
<menulinks>alpha</menulinks>

<jivepattern>^(.*)\s\((.*)\)$</jivepattern>
<menudata>
select albums.id,albums.title,substr(albums.titlesort,1,1 ) from albums
order by albums.titlesort asc,albums.disc asc
</menudata>
<menu>
<id>track</id>
<itemtype>track</itemtype>
<itemformat>titleformat</itemformat>
<itemformatdata>TRACKNUM. TITLE (ARTIST - DURATION)</itemformatdata>
<jivepattern>^(.*)\s\((.*)\)$</jivepattern>
<playtype>all</playtype>
<menutype>sql</menutype>
<menudata>
select tracks.id,tracks.title from tracks
where
tracks.audio=1 and
tracks.album={album}
order by tracks.disc,tracks.tracknum,tracks.titlesort asc
</menudata>
<menu>
<id>trackdetails</id>
<menutype>trackdetails</menutype>
<menudata>track|0</menudata>
</menu>
</menu>
</menu>
</menu>
</custombrowse>

Now my question: Why does this work:
<itemformatdata>TRACKNUM. TITLE (ARTIST, COMPOSER - DURATION)</itemformatdata>

but not this:
<itemformatdata>TRACKNUM. TITLE (ARTIST, PERFORMER - DURATION)</itemformatdata>

PERFORMER is a custom tag scanned into the database by CustomScan. What gets displayed is PERFORMER as a string but not its value, e.g.:
1. Diamonds on My Windshield
Tom Waits, PERFORMER - 3:08

Am I missing something here? Thanks...
Thomas