The upper part shows that "can't/couldn't" are definite: it says 99% true. But "may/might" imply "maybe", which on the chart is less than 50%.
So "I could go to London" means "maybe I will go to London", but "I can't go to London" means "I definitely will not go to London".
The pair of examples at the bottom is supposed to show the difference between how "can't" is used and how "couldn't" is used. Is this diagram from a textbook about BE (British English)? It is not correct in AE.