Thanks!
About your question, let me explain it this way: faced with two possibilities (ex.: does the monster notice me, yes or no?), there is one that would be worse for your hero, but easier to happen (it does notice me), thus LIKELY, and one that would be better for your hero, but harder to happen (it does not notice me), thus UNLIKELY. The way it is, it is more probable that the LIKELY one happens, but you can try to push your luck into the less probable one, risking a MISFORTUNE (ex.: it noticed me even before I noticed it and is gonna ambush me). I hope it's clearer now!