Just like the Raptor 700, it is a 686. If I remember right.There are no 102.5mm pistons.
719.1xxxxxx is the calculated displacement. The stock 660 is 659.7cc. Craycraft used to sell their 686cc kits as "690" and 727cc kits as "730". A polaris RZR 800 is 760cc.
They "WHY?" varies. Aesthetically we like round numbers. Also, rounding up sounds bigger, our culture views bigger as inherently better. Therefore if something sounds bigger, it's worth more.
A calculator and a chart here may help shed some light
741's are routinely called 740, 762 are often called 760. 779s are always called 780s.