Hi Sebastian,
I'm not sure if I understand your problem entirely, but have you checked the assigned age in the reference folder for the 91500? The age of your primary reference material will be normalised to this value.
On a side note, in one of the recent IAG G-Chron inter-lab comparisons, the organisers noted that different labs uses a range of ages for 91500 zircon. Some use the 206/238 age (1062.4 Ma), others the 207/206 age (1065 Ma), and others use more recent data. I would suggest that you confirm that the age and 206/238 ratio in your reference file are coherent.
Cheers
Pat