Default volume for normalization is 89.0 dB. You can see it on status line. You can change it to another values then 89.0 dB at Options-Normalization-Target Volume.
It is always good to make undo track volume, before going with another target volume if files are already normalized.
Zortam is using mp3gain (http://mp3gain.sourceforge.net/faq.php) for the normalization process.