Manifold Vacuum Advance what am I not getting?

I’ve seen side-by-side comparisons of ported versus manifold gauges while driving. They don’t always do the same thing everywhere. I was surprised that there are differences. I think they’re minor though, but they are different.. But for the sake of this conversation, I’m assuming that they’re very close and that the only differences tuning.
And to be more explicit the big difference is at idle from there on out similar not identical….