Seriously, why do you think I don't know the difference? Be specific.
Because most people who ask questions here don't.
And how is it that Play Store prompts me to uninstall apps that Application Manager says are in external storage? Why does Marshmallow contradict itself?
Prompts you to uninstall apps, or lists them as Installed? All that means is that you once installed that app to a phone that was on this account - even if the phone died and you threw it away years ago, it's still listed as Installed.
If you uninstall the apps, you uninstall the pieces moved to the SD card, and you uninstall the links. That should free up a little internal space, but not that much. It's almost a Catch 22 situation - moving them to the card doesn't free up much internal space and uninstalling them after that doesn't free up much internal space.
Don't blame it on Android or Google, though - Android was never designed to run apps from the SD card. That was a bug found by someone back around Eclair, and it's been exploited ever since. (And I suspect that Google is afraid to fix it, because it would appear to break so many phones.) But the rule of thumb now is the same as it was in Cupcake (1.5) - get a phone that has enough storage to last until you get your next phone, because storage in a cellphone isn't replaceable.