The Mythic Raid drop is guaranteed to be at least 880. We flip our imaginary coin (again, these are NOT the actual odds...). 50% of the time it'll come up Heads (woo, 885!), so we flip again.
Again, half of the time we get this far (25% chance total, now) it'll come up Heads again (890!).
And so we flip yet again, and yet again half the time we make it this far, we'll get yet another Heads (12.5% total).
That bumps the item to 895, which is the cap, so we stop flipping coins and the item ends up at +15 over the base 880, which makes it Titanforged.