dont be confused its simple, you want low gi carbs (complex carbs) throughout the day, this is because the gi rating is how fast it is absorbed, if it is low it will be absorbed slowly and this is what you want throughout the day. however postworkout you have just depleted your glycogen stores for energy and need to replenish them for your next workout immediately after your workout b/c at this time your body has its best "window of oppurtunity" you could call it, this is why you want simple carbs (high gi), they are absorbed quickly which is what you want postworkout.