You'll write different bugs in different languages. You can't leak memory in Java (you can, but it's very hard), for instance, as you can in C. In Python, I often write code that barfs when it gets an object of a wrong type because parameter types are simply not enforced. The bugs I write in Lisp are entirely different from the ones I write in C.
But if we agree that Ada reduces one kind of bug compared to, say, C, then we'd also have to come up with a class of bugs which Ada makes easier to write, in order to say that the number of total bugs stay constant while one kind decreases.
I can't come up with such an example.
You're right in that things are rarely black and white, but when it comes specifically to bug prevention, I think Ada is a strict improvement over many alternatives.
We'll always write bugs.