I was told by someone yrs ago. Pokeballs have a catch rate, meaning that every time you start the game to get a Pokemon. The first ball you use has a predetermined odds of how effective it will be in getting the Pokemon each time you start it. Take for instance catching Uxie. Doesn't run, or anything. Save in front of it, and battle. Everytime you try and get it, the Pokeballs you throw don't react the same each time. Battle it, get it in red toss an Ultra ball. Breaks out with no wiggle. Second attempt, it might wiggle twice. I've heard of people getting legendary's with Pokeballs, Great balls, and Ultra balls. It can be done on either one. Just different odds of it working depending on the Ball used, and the odds the ball has of working. In Diamond i caught Uxie with one Ultra ball. Azelf it took 3 to get him. Mesprit required a lot of restarts.