With spoofing disabled it returns:
IBrowse/2.3 (AmigaOS 3.9)
Enabled the string changes to:
Mozilla/4.78 [en] (Win98; I) IBrowse/2.3 (AmigaOS 3.9)
According to posts from the IBrowse team on the mailing list this was tuned for best compatibility both for getting in to sites and for sites that return pages tuned to a specific browser. I believe that future IBrowse versions will have more flexible spoofing.
HTH
Robert