If Focus effects were to increase in % over each consecutive iteration, spells would not be able to continue to upgrade at anywhere near the rate they currently do. They'd essentially take over as the primary form of caster upgrades. If the spells and focus effects continued to scale at the current rate you'd end up with astronomical power games over very short level ranges. There's certain a sizable amount of power that you can expect to gain by increasing in level, but when that increase gets too large is when you start having problems with balancing players against the world. We saw a lot of this in Kunark and Luclin when players were able to kill trivial Mobs for the same exp that challenging mobs used to provide. Old content should continue to get easier, but there's a certain level of power increase that the game can handle and stay healthy.
Consider vocal dynamics as the changing volume levels in which a vocalist sings during a song. These changes can be detrimental to your mix but they can also be beneficial, it all depends on the intent of the song arrangement and the quality of the singer. For example, a vocalist can raise their volume at a critical part of a song and that adds to the emotional impact at that time. There are also the tell-tale signs a singer isn’t comfortable singing a song when they belt out the chorus but their volume drops significantly when it comes to the verses. You might even hear a bit of hesitation in their voice. You can’t control everything.