If you pick one you’re 5% likely to pick the right one and be 100% correct.

You’re 95% likely to pick the wrong one. Then you miss the conspiracy that is true and you believe in one that is not true so 18/20 = 90% correct.

Average out the probabilities and it should turn out to be something like 90.5%.

Edit:I may be off on the maths now that I think about it.