Because your PRIV does not entitle you to purchase First class tickets.
Just because it's an excess doesn't change the fact you would be obtaining a ticket for 1st class travel.
Although I'm not entirely sure on the safeguarded staff, here is an example with a TOCNE Standard Class Priv.
Liverpool-London SOR priced at £57.75
I decide I want to go first class so ask the TM for an upgrade to 1st. He informs me my Priv does not entitle me for 1st class priv rate tickets. Therefore the excess would be between the discounted (priv) fare and the full public fare of the FOR. Therefore an excess of £312.25 would be payable (£370-£57.75) .
My only question would be does the 'box' count as a Standard Open ticket priced at Priv rate? Therefore, are you able to excess in the first place? What is this box worth as a price?
On another note, some managers work very hard. They are rewarded with 1st class travel facilities as part of their benefits package. Although I'm not saying staff with standard privs don't work as hard- it would be unreasonable for every member of staff to travel 1st class as there is far less capacity in the superior accomodation. The only way I can think of distributing these passes fairly is to go on 'grade' as per the present system. Therefore that is why I think staff should not be allowed to upgrade unless they have full 1st facilities.
I'm pretty sure I said mine didn't??