The statement after decreases has to get strictly smaller in each loop and always be non-zero. But does it have to reach 0? Does it have to get smaller by one?
As stated in the JML documentation, decreases (you can also write decreasing) means that an int or long expression with that specifier "must be no less than 0 when the loop is executing, and must decrease by at least one (1) each time around the loop."
So it may or may not reach 0, but can't get smaller than that. Also, it has to get smaller by at least, but not necessarily exactly one. Note the example in the documentation for a more precise explanation.