It is a pretty safe assumption that the incident meter is "right" as far as the ISO standard.
When using any target in the scene it is important that it be used in a consistent manner and that you know what the offset is.
The grey card, as Bill indicated, isn't necessarily a match for the ISO standard.
Take a reading with the incident meter then with any target in that same spot, take a reflected reading with your spot meter. The difference is the offset you need to use for that target.