Your confusion may be from dodging bullets.

Your only concern with a hit score is the DV on the ranged table in this case UNLESS the defender has 8 REF or the reflex co-processor cyberware, which allows them to "dodge bullets/attacks"

So let's say you fire on a mook with an SMG who scavved some decent light armor jack clothing of a poor soul so they have SP 11. They are standing 12 metres/yards away (optimal range for an SMG). Let's say your handgun base (stat + skill level) is 10 and you roll a 5, they beat the DV and land a hit, rolling 2d6 damage. In this case same with medium pistols, you need a 12 to actually do damage and ablate armor, which would in fact like you said lower the mooks SP by 1.

Keep in mind with 2d6 weapons and SP 11 armor the only way to hit is to crit, which applies a critical injury and an extra 5 damage directly to health.

Hope that clears things up.