I see this chart quoted a lot but I looked at three varieties and they are all listed differently than everywhere else I’ve looked.
Gala is listed as group 3 but these two sources list it as group 4
Crimson Crisp is listed as group 5 but it’s usually listed as group 3 in the former source and here: https://www.foodforestnursery.com/growing-guides/fruit-trees/apple-trees-growing-guide/apple-pollination-chart-3/
I only checked those two and Goldrush (group 4 vs 5) but they are all different. Either Grampa is wrong or the rest of the internet is.
Edit: Grampa’s chart also lists mid-season bloomers (group 3) as capable of pollinating both group 1 and group 5, which would effectively make half the apple varieties universal pollinators. There’s a note at the bottom that says neighboring groups can give better results but does not contradict the main chart.