The pump pulley has adjustment more to aid fitting of the belt rather than having to change timing but every belt will be slightly different so this will allow for those differences without changing the more critical cam to crank timing.
If the injector pump is a little bit out it just wont start but it the cam is out the wrong way then you break stuff.
https://www.aulro.com/afvb/images/im...008/10/932.jpg
I think you will see there that there is no adjustment on the cam pulley on a 300Tdi, the adjustment is on the pump.

