According to Bulbapedia:
Since Generation V, when a Poke Ball is thrown, a special type of capture that is much more likely to catch a Pokemon, called a critical capture, may occur. When a critical capture occurs, the Poke Ball will make a high-pitched whistling sound as it is thrown, then pause in mid-air, and shudder before it drops down to the ground, shaking only once on the ground before the Pokemon escapes or is caught.
When you're attempting to catch a wild Pokemon, usually the poke ball (or whatever ball you use) will shake multiple times before the Pokemon is caught. These shakes are called shake checks, and they each use a mathematical formula to determine whether the Pokemon breaks out of the poke ball or not. A Pokemon is successfully caught if it does not break out after three shake checks.
Well, a critical capture skips the first two shake checks. Only one shake check is needed to pass to determine whether the Pokemon is caught, sharply increasing the chances of you catching it. However, the Pokemon could still break out; a critical capture is not a guaranteed catch.
So, if the ultra balls you used on Kyurem and Zekrom only shook once instead of three times and whistled when you threw them, that means they were critical captures. If not, then I suppose you were pretty lucky.
Source
Hope this answers your question!
