The Major/Minor thing, as said, is not IDPA and is for USPSA.
Also, it's not anything that requires and extra shot.
The way it works is that there are various scoring zones on the target: A, B, C, and D. A is center of mass and head - B is rest of head and C outside of center. D is outermost area.
If shooting Major Powerfactor, A's are worth 5 points, B and C are 4 points, and D's are 2 points.
If shooting Minor, A's are still worth 5, but B/C is only worth 3, and D's are only worth 1.
So if shooting minor you lose some points on bad shots, but you can recover quicker from the recoil. Single-stack also has an even more significant tradeoff between the divisions - not only do have the tradeoff above, but in Single-Stack you use 8-round mags for Major PF, but for Minor you can have 10 rounders.
Also, if shooting in Production division (which is where many Glocks are shot), due note that in that division there is no distinction - everyone is scored minor anyways.
In general, I shoot production where 9mm is the clear choice.
my uncle died trying to defend himself with a 9mm, putting 1 round in center mass, but it unfortunately didn't stop the guy.
Not to get into self-defense ballistics here since this is about competition, but without other data you can't really fault 9mm there. 9mm failing to stop a threat doesn't mean the 9mm was inadequate. Depending on the situation a bigger round may have not done any better.