It's really not that confusing if you think about it in simple terms.

The distance of IAD-ZRH-BOM is 8225 miles one-way, 16450 miles return.

Looking at the required miles chart, when the roundtrip number of miles is between 14,001 to 18,000, the required number of miles in Y is 65,000.

The required number of miles will not change whether you change at ZRH, VIE or FRA.