Enderman
Enlightened
This is simply due to better manufacturing and battery improvements, there is nothing magical about the 21700 size.More about the BMZ battery pack using 21700 cells:
So, this Sony/BMZ 21700 cell has 60% more capacity and 400% more discharge current than a 18650 cell. Sounds like a big upgrade to me.
It's just a size between 18650 and 26650.
If they used this improved manufacturing for 18650 or 26650 cells you would also see more capacity and higher discharge.