It’s not any “header of the branch” that matters.
The “Event” branch is an “Int_t Event_” and it keeps the actual number of elements (different TTree entries can have different numbers, from 0 to “kMaxEvent = 1” in your case) in the “Event.Weight” branch which is a “Float_t Weight[Event_]” variable length array (and another variable length arrays related to another “Event.*” branches, as far as I can see). I assume you got the maximum number of elements “kMaxEvent” from the source code generated by the TTree::MakeClass (i.e. you got something like “const Int_t kMaxEvent = 1;” and then “Float_t Event_Weight[kMaxEvent]; //[Event_]”).
If you are familiar with the “Delphes” tree, could you maybe have a look at: Problem generating dictionary