I don't know - maybe you can google android architecture memory management with better luck than me. Sometimes they do tricks with sharing use of memory anyway, which you cannot detect by the settings numbers.
Gingerbread has been called more memory intensive than Froyo. Maybe it is to support the below stuff quoted from wikipedia. I certainly see an improvement in the gaming parts. Has anyone tried the voip?
2.3 Gingerbread (open-source) refined the user interface, improved the soft keyboard and copy/paste features, improved gaming performance, SIP support (VoIP calls), and added support for Near Field Communication.