Pokemon-go – Are Pokemon’s moving or standing where they spawned in map-mode

Borrowing from @Codestrife's answer here

The rustling leaves usually indicate that there are pokemon near that area. Going and walking around near that area gives you a higher chance to encounter Pokémon.

Keep an eye out for rustling leaves, hinting at where critters may be lurking and simply hang out in a suspected region for the increased chance of an encounter. - Source

Therefore, the reason you don't always see Pokémon in rustling grass is because there is only a higher chance to encounter them there and not a guaranteed chance of encountering them. You just have bad luck when you don't encounter them.

When talking about IV vs CP,

Combat Power is only an average of your Pokemon’s stats and is only a rough estimation of a Pokemon’s strength. -source

There are 3 IV's that we know of; Attack, Defense, and Stamina. Defense and Stamina are important when it comes to defending in gym battles, and Attack and the move you are attacking with's base damage and attack speed are important for attacking gyms.

So for defense of gyms, a Pokémon with a lower cp but higher Defense and Stamina may be a better pick than a higher cp Pokémon who just has really high Attack. For attacking, you will want to prioritize Pokémon with higher attack and lower defense, as you can always dodge to avoid taking a lot of damage. See here for more info.

Also, see here or here for an IV calculator.