Its a combination of a number of rules that taken by themselves make a lot of sense, but in combination are absurd. Here are the relevant rules:
Vehicles are immune to Stun damage (ie flashbangs, tazers, the Stunbolt spell, etc.)
When a grenade goes off near a wall, the blast reflects back from the wall. So if you are stood next to a wall, you will be hit once by the blast, then once again by the reflection of the blast.
Stun grenades do 6DV of stun damage uniformly across their 10m blast radius.
The average metahuman can take 10 points of stun damage. Any stun damage after that is converted into physical damage, which they can take another 10 of before they fall over, and on average another 3 points kills them outright. So to kill an average metahuman with stun damage, you need to hit them with at least 23 points.
All of that seems pretty sensible, until you throw a stun grenade into a car and close the door.
Ive just done the maths, and based on a car interior of about 2m x 2m x 1m (which is probably a bit on the big side, but makes for easier sums), a person in the middle of the car would end up getting hit 41 times, for a total of 246 stun damage

For comparison, a small Nuke would do about 130 damage at the centre of the blast.