fire value for mortars

[wololoh]

Hi,

I was playing around with PzB Normandy, and found that the mortar units were particularly ineffective, so looked a little further into it.
Example: American mortar of 4 guns with morale C (soft attack 7) fires at infantry in clear hex (defense 16). I expect the fire value to be 17 in this case.

Actual results are (from fire dialog):

- versus hex with 33 infantry: fire value 2 (basically nothing!)
- versus hex with 99 infantry: fire value 8
- versus hex with 198 infantry: fire value 17 (-> finally the expected value!)

So the expected fire value is only reached when firing at a fully stacked hex. Most importantly, the fire value literally goes to zero against "small" (actually quite common size) units.
In the manual I see the following: 

Stack Fire – a modifier applies to Direct fire and Indirect fire from half
the stacking limit (1.0) up to maximum stacking limit (2.0). This
represents the higher casualties due to the increased density in a hex.

However, what actually seems to happen here is that all fire gets a negative modifier (ranging from 0 to 1), unless firing at fully stacked hex. For this mortar fire, the fire value literally dwindles to nothing based on stacking value of the target hex. Is this intentional / is this the case for all ranged fire?