the screen resolution items encapsulate internal system parameters.

Thus, if your display or a detail of the operating system in that area 
changes, so will the effect of the encapsulated parameters.

While I haven't checked, I think it's quite possible that there have 
been changes in Leopard in general or in 10.5.2 specifically.

And while the item library items (or in System > Screens) are always 
generated freshly from your current setup, once you put such an item 
in a customized menu, a snapshot of the parameters will be used and - 
unlike the freshly generated ones - can become outdated as system 
internals change.

Best regards, 
Felix Schwarz