firstly, you're misinterpreting the values.
lowQty:
Minimum quantity spawned at beginner/experienced
highQty:
Minimum quantity spawned at superhuman
(minimum quantity for veteran/genius is the median of the previous two values)
dQty: d stands for delta. this is a random variance that adds from 0 to dQty extra aliens (all difficulties)
extraQty: functionally identical to dQty, stacks with it.
secondly: it's converted directly from the original exe.
there's no logically consistent pattern, it just is what it is.
https://www.ufopaedia.org/index.php?title=Large_Scout#Alien_Deployment for reference.
see the deployment for soldiers: 1->6, 2->7, 3->8. exactly as described by these numbers.