It's similar (in a way) to the difference between V maxROC and V obstacle clearance.
In one case, you are merely trying to reduce fuel flow (kg/hr) to minimum so you can last a max. number of hours. You don't care how fast you travel; you are simply very near the min. drag speed.
In the other case you are shooting for the best distance traveled per kg of fuel. Speed obviously enters into the equation; you want the quantity
Knots (NM/hour) / Fuel flow (kg/hr)
...to be at a maximum.