The description of HoldForm is:
HoldForm[expr] prints as the expression expr, with expr maintained in an unevaluated form.
The trace is displayed as:
{{2 3,6},6+4,10}
But in order that it not be evaluated, the actual code is:
{{HoldForm[2*3], HoldForm[6]},
HoldForm[6 + 4], HoldForm[10]}
which is what the tree displays.
