[Zope] Difference between getItem and restrictedTraverse

[Sven Hohage]

Hello,
I build a frameset with a subnavigation ->
<dtml-if "REQUEST.has_key('obj')">
 <dtml-with "_.getitem(obj)">
 show the content of the obj
 </dtml-with>
 </dtml-if>
 When a user Mr.Smith with a role VIP tries to log in, the
subnavigation-frame is closed (though Mr.Smith can see the content frame
with the same permissions) and Zope throws an exception:
"Anonymous user is not allowed.The user would need to have the roles
[Manager,VIP]."

When I switch <dtml-with "_.getitem(obj)"> with <dtml-with
"restrictedTraverse(obj,None)">
Zope accepts the VIP-Role and shows the frame.
Hmmm..
I'm confused about the reasons for this behaviour and therefore about the
differences between getItem and restrictedTraverse
Thanks
Sven