Myth Parallel

GURPS Infinite Earths provides some useful concepts and terms to describe them.

Maka Tanka is an "high-inertia parallel" up until the Indians complete their gateways and abscond to Maka Tanka from Earth.

But after that...

Inertia is broken and changes start to stack up.

Some examples:

Before the decoupling, events in this world are very similar to our world despite the existence of something, like magic, that should be very disruptive and world altering.

You have to have answers to why Lincoln isn't president on April 16, 1865, despite effective healing magic.

Well...  In my world, Lincoln is alive.  He's no longer himself.  In effect, Wilkes-Booth's bullet crippled Mr Lincoln's brain.

Great healing restored all of his hit points, but didn't put his brain back the way it was.  Regeneration is the spell that's required and you only get one chance at that.  The surgical mage failed his roll and Mr Lincoln stopped being himself from then on.

He's still alive, and a completely functional person, he's just lost most of his memories and is no longer president.

This is going to come up again for the McKinley.  Rather than dying from gangrene days later, he's healed moments after being shot.  He is so shaken by the assassination attempt that he resigns at the behest of his wife and retires from politics.