Ball bounce strength is random, sometimes it slows down, sometimes it speeds up, so that's kinda confusing.
Yes, that is the intended behaviour. The ball chooses a random velocity, with respect to the side it's on, after every bounce with any of the paddles.
In my game, only the former is done. But I do admit that the range is a bit radical and I could do something to make it only change slightly with respect to the min and max allowed velocity, instead of being completely random within a range.