My legs are grey. My ears are gnarled. My eyes are old and bent, so I'm getting rusty on my PT6 theory in my old age, but......
Were your platform a twin power plant, then my educated guess would be to prevent the potential for an overtemp at low idle speeds on the first engine now running at idle, during the run-up of the second engine start sequence.
Were the NG on the initial start engine running at idle to drop noticeably as a result of a high load created during the 2nd engine start, then the governing protection offered by P3 > PY bellows would be reduced and the FCU's start acceleration fuel scheduling may kick in additional fuel, but with a declining mass air flow past the fuel nozzles in the burner can, may result in the risk of a surge in ITT, as such the OEM sets a much lower idle ITT limit with plenty of margin.