Battery Cabling for a DC Power Plant
The answer to your first question is that you have to work out the 5hr discharge rate and then, using the resistance of the cable, calculate the voltage drop - this answer is compared to the answer from the 2hr calculation based on the ampacity : then take the larger of the two...

Based on your comment:
The results will differ based on the cables used for each calculation : if you use the same cable then as you mention the 5 hr average will be lower than the 2 hr average but the 2 hr average will need a larger and more expensive cable...