The boost clock depends on other factors(power/temperature/power usage. ) as well. For most users core use will be the primary thing that effects clock speed. Under single core load you should reach full speed, but as the number of cores loaded increases the boost clock tends to drop.
Modern video cards do a similar thing.
As said above, you can set a clock rate for and even under all multi core load you will get higher speeds. You need an unlocked(multiplier unlocked) cpu to take advantage of this.