The explanation is in a literal sense, . . . just exactly what it says.
Try to think in terms of your whole left index finger as being a finger of 100% in quantity.
The length from the very tip to the first joint would be the first 33%, . . . from the first joint to the second joint would be 34 to 66%, . . . and from the second joint to the knuckle would be 67% to 100%.
Everyone who shoots, uses their trigger finger somewhere in that first 33%. If you are left handed and are shooting to the right, you may only have 15% of your finger into the trigger guard. Try putting more finger in there, . . . say 20%. Still shooting right? Put more finger in there, . . . say 30%.
When you find where you are beginning to shoot to the left, . . . you have too much finger in there, . . . back out some of it.
What BillCA said: "When shooting a SA style auto, the center of the fingertip pad should be on the center of the trigger," is generally a correct statement for most shooters, . . . especially if they are shooting targets, one handed, such as NRA competition, etc. I am one of those people, . . . definitely just the pad for one handed X ring shooting.
On the other hand, . . . if I am shooting rapid fire, . . . or point and shoot type exercises: I can only shoot accurately if my first joint is dead square in the middle of that 1911 trigger. In other words, 33% of my finger is sticking out of the trigger guard.
Work with it, . . . try some dry firing exercises, . . . you will be amazed at how much difference just a little adjustment makes.
May God bless,
Dwight