Hi Giuliano,
If the project is not scheduled within the selected dates, those will not appear logically in results. If you think that you are not seeing the projects which have been scheduled within the selected dates, you can raise call with Ariba by giving them examples as I don't see any reason why those should not appear.
While I am writing this, I have a feeling that Ariba deal with future transactions differently. In P2O, there is "Delay purchase until" and if this is filled to some future date then the PR never gets into approval as Ariba understand that the transaction is for future so no doubt the report might have been designed to ignore scheduled projects for future dates though you will get clear answer from Ariba on this.
Regards,
Ravi Pachauri