It certainly is a game of compound interest! On top of that the system has three features that interact to give it its depth:
1) There are several opportunities in each game to sacrifice your rate of growth to obtain a temporary advantage in present value (ie. cutting workers to rush).
2) Because present value is multidimensional (you can't measure the value of a Zergling directly against that a Mutalisk), you can sacrifice your rate of growth or your present value in order to obtain a temporary advantage in what kind of value you have (ie. cutting units or workers to tech).
3) Information asymmetry. While trying to gain these mathematical advantages, one has to act on information that is increasingly out of date. One can not usually predict the opponent's exact present value and rate of change, but can only estimate ever-widening bounds on their values.
1) There are several opportunities in each game to sacrifice your rate of growth to obtain a temporary advantage in present value (ie. cutting workers to rush).
2) Because present value is multidimensional (you can't measure the value of a Zergling directly against that a Mutalisk), you can sacrifice your rate of growth or your present value in order to obtain a temporary advantage in what kind of value you have (ie. cutting units or workers to tech).
3) Information asymmetry. While trying to gain these mathematical advantages, one has to act on information that is increasingly out of date. One can not usually predict the opponent's exact present value and rate of change, but can only estimate ever-widening bounds on their values.