A hit with Indirect Fire scatters 1d10-BS meters. A miss with a Comrade assisting scatters 1D5. This means that until a character reaches BS 5 a hit can scatter further than a miss, and as the standard Mortar ammunition (Frag Round) is Blast 5, a miss will always hit the target, whilst a hit can scatter out of range.

I'm assuming its supposed to be 1d10-DoS and 1d5-DoS

That's…kind of disturbing actually. This should be brought up and possibly sent to the proofreading section.