Question: He was on the borderline of red and yellow health and was paralyzed. For the regi trio I had to use about ten each.

Answer: 
The Premier Ball has the same catch-rate as a regular Pokeball. When you throw the ball, the ball's catch-rate, combined with how 'hard' it is to catch a certain Pokemon is plugged into a formula along with a few other things like current Health to determine whether you caught a Pokemon. 

Your chances were very low (a low catch-rate ball combined with a hard to catch legendary). For some perspective, a full-health Regigigas has a [0.4%](http://bulbapedia.bulbagarden.net/wiki/Regigigas_(Pok%C3%A9mon)) chance to be caught with a regular Ball. While yours was injured and paralyzed, the catch chance would still be very low.

Thus it is not *impossible* to catch a Pokemon this way. You just got very lucky.
You only got extremely lucky.Regigigas' catch rate is pretty low,but since premier balls are only there to give you a reward for buying 10+ pokeballs in one visit to the PokeMart,they just have the power of a regular pokeball.