It’s not plucked out of thin air. As I expressly stated, it’s based on what
@SWP's back reasoned in his post.
If you have such poor comprehension skills it’s little wonder you struggle to evaluate likely outcomes with any apparent care or skill.
Your reasoning appears to be rooted in what happened in 2016, since when, as has been posted many times, a number of factors have materially (some overwhelmingly) changed.
My figure, based in that logic, places Trump’s prospects as less than likely (so concomitantly less than 50%) but realistic and plausible, hence the figure of 33%.
I accept its quite a crude and simple calculation, but is in direct response to yours, which is misconceived and flawed.