So what's up with that guaranteed Ultra Ball shake?

I don't have a gif of it unfortunately, but I'm sure you guys have seen this. When you chuck an occasional Ultra Ball (Dunno if it does this with other balls), it'll give an extra shake in the air before falling to the ground, which then twitches once and clicks instead of three times. Do we know why it does this? I mean, I dig it, but why does it do it?