OS X 10.5 on macmini3,1 model?
The usual Apple way is that a set of generic OSX disks only support all machines that come out before that OS (How would any know what a new machine might have changed). Machines released after an OS release will include boot disks that can be loaded by that machine but earlier versions might not.

Thus I suspect that your mini is unsupported for Leopard or people bought it earlier with a leopard version for that machine.