Gas gauges on A-bodies

I just had a thought (yes, I do have them occasionally).
The original fuel gauge and sending unit ran on +5VDC, the new gauges are running on +12VDC. I wonder if that has any bearing on this. Yes, I understand that it has nothing to do with the sending unit resistance setting, but I wonder if this could be a factor..

As I sit here and think about it, it makes absolutely no sense that an after-market gauge should not work with a after- market sender. It is simply electronics and it should work, But it does not, I just do not understand why