I think I have run into another bug in v. 3.1 of assign. This one is a weird one and not consistent. In the Assignment Inspector for some sites the displayed position of the peak is not the actual position but rather some other value (average over all sites?). When a specific peak is selected in the Assignment Inspector, you can see the correct position in the Peak Assigner but not in the Assignment Inspector. This bug is inconsistent because for most sites the displayed position is correct; it is only some sites that display this behaviour (edit: it seems to be a problem only for nitrogens).
Here is an example with the value in the Assign. Insp. on the left and value in the Peak Assigner on the right.