I think for the stats such as luck, spirit, power etc they randomly increase when leveling up. You could win one battle and get +2 on every stat and then load your game and try again only to get +1 on all or some. It's pretty random and very frustrating if you want to naturally max out all your stats.

I think the same thing happens for HP and MP when leveling up. I know Barret can reach 9999 HP at level 99 without a HP plus materia but leveling him up to Level 99 is now guarantee that he will.

Also I think Cloud's stats at the very beginning before any fight can vary from game to game. That's why there isn't a dead-set way to send Cloud into Lucky 7's just by equipping this and that Materia as every body's level 99 Cloud's will likely vary in someway.

I might replay the start of the game again just to see if what I just wrote is true or not lol.