Without using complex mathematics, please explain why best range airspeed is less than normal cruise airspeed.
Normal cruise is a compromise between being fuel efficient while getting to your destination at a reasonable speed. It all comes down to drag; the faster you go the more parasitic drag you pick up and economy suffers.
Just like in your car - if you wanted to squeeze max efficiency (range) out of a trip you could drive at 55mph on the freeway. In fact you could even make it law! But, people are generally willing to burn a little more fuel to get there faster.