It is possible that it is always a 75% increase in an attribute per level of the gem, and that the +2 from the Chipped [[Ruby]] is increased by 75% to 3,5 on the Flawed [[Sapphire]] and then rounded (to the closest number without a decimal) to an even 4. The +4 from Flawed [[Sapphire]] increased by 75% equals 7, like we can see on the [[Emerald]]. All of this is pure speculation and highly likely to change, since itemisation is one of the last things that gets done in the development of the game.