Private means private
Mark a task private and it is yours alone. It sits in the shared list, in your view of it, and nobody else in that list can see it.
Features
A shared list can still hold something private
Sharing a list usually means giving up your own space in it. That is why people keep a second, secret list somewhere else.
One list instead of two
You stop keeping a shadow list for the parts that are not everyone's business, which is where things get forgotten.
The line stays where you put it
Nothing changes a task's visibility on its own. What you marked private stays private.