Possibly there are engineering works on the day you want to travel and the timetable has not been finalised yet.

Which departure time would you like to book?

Most often the LIMIt on book ahead is 3, more often 2 monthes. That does not mean its always exact 3 month advance-every now and then they release several days or weeks. ITaly is much noted for tardy sloppiness in this. And the fears that all newbees seem to have for ´full´ trains so long ahead is complete nonsense.

Thank you! I was planning to take the train around lunchtime-13.39? Could be some engineering works,I was just puzzled !

There are indeed engineering works going on around that time. For the 13:39 departure with EC 39, this means the train probably ends at Domodossola (its first stop in Italy) and you'll probably need to change to a regional train. This would also mean you don't need a reservation, since that is only mandatory from Domodossola.

On the Trenitalia website, none of the EC trains are bookable on 20 September, which confirms that the timetable is still not final. I'd say, check again in a few weeks’ time. Once the trains appear on the Trenitalia website as bookable, their timetables should be fixed.