Per the title, some mortar shells have wildly different radiuses for "exploding as a projectile" compared to "exploding when they get destroyed on the ground."
I've attached an image I created in dev mode comparing these values for the smoke shell, which has an explosive radius of 7.2 when fired (red tiles in the attached image) compared to an explosive radius of 11 if you just set one on the ground and shoot at it or otherwise damage it (blue tiles in the attached image).
This is defined in Items_Resource_Shell.xml on:
EMP Shell
Line 159: <explosiveRadius>11.9</explosiveRadius>
Line 189: <explosionRadius>8.9</explosionRadius>
Smoke Shell
Line 211: <explosiveRadius>11</explosiveRadius>
Line 239: <explosionRadius>7.2</explosionRadius>
Firefoam Shell
Line 262: <explosiveRadius>9.9</explosiveRadius>
Line 296: <explosionRadius>5</explosionRadius>
To recreate this issue, simply start a new game, spawn two mortar shells of the appropriate type in dev mode, then fire one of them out of a mortar at a target and detonate the other one by applying "Damage 10" dev tool until it pops.
The actual explosive shells (High Explosive, Incendiary, and Antigrain) all have matching explosive values.
Good catch, I will put this in as a bug report. Thanks!