You're forgetting this fictional game is set 38,000 years in the future. Small explosives from just 200 years ago can't do what modern explosives can, so why can't small explosives have advanced in the last 38 millennia?
Bad logic. An explosive from 200 years ago does the exact same damage as a similar explosive dies today, and as a similar explosive will do 38,000 years in the future, because physics is the same. Explosions are not cylinders, they are spheres. An explosion blocked by a wall will be blocked by a floor.
The in-game issue is that
GW failed to replace rules for multiple level structures in 7th that were present in previous editions of the game. And common sense (a very uncommon thing) lets you see under an opaque blast mark by simply checking the edges from above, and noting the stuff in the middle.
Per
RAW, blast are Cylinders that ignore Flyers, not spheres that hit everything within the sphere not masked by a wall or floor. Way to go GeeDub.
SJ