I was wondering the exact same thing. Here's my interpretation:
Small explosives spaced throughout the exterior of the vehicle prematurely detonate/deflect incoming fire, reducing the AP value of attacks by the smart armor’s rating.
Every time a heavy ballistic weapon or explosive (usually any weapon that does more than 10 DV) hits a vehicle with smart armor, roll a test using the smart armor’s rating as the dice pool. Every hit is added to the firing weapon’s AP value (thus rendering the smart armor less effective).
First of all, I agree with you that they seem to have mixed up "add" and "reduce". As it is, smart armor would help the opponent, since AP is usually negative if it favors you...
Now, I understand it this way: Smart armor is supposed to be less effective against heavy weapons with a base damage of at least 10. If that's the case, you roll with (smart rating)d6. The hits get "added" to the AP rating, which is basically equal to the smart armor being (temporarily) reduced by the hits (which would make more sense to me). That doesn't mean smart armor is helping heavy weapons, it's just that its protection is less effective. Smart armor cannot increase the AP to more than its rating, because the number of (rating)d6 hits is always equal to or less than rating.
Example: Doberman with normal armor 9 and smart armor 3.
Ganger 1 shoots with Ares Predator, 4 net hits => DV of 9 AP-1 => AP gets "reduced" to 0 by smart armor => no damage thanks to smart armor
Ganger 2 shoots with Panther XXL, 2 net hits => DV of 12 AP-5 => you roll 3d6, get 1 hit => enemy gets one "more" AP, ie. AP-6 => smart armor "reduces" it by 3 => AP-3 (To me it seems easier to just reduce the rating by the hits, ie. 3-1=2 & 5-2=3, so AP-3, but yeah...)
Also, I don't think the rating is decreased by the hits rolled. It is only ever decreased when you glitch. Furthermore you can only use smart armor rating times then it must be replenished.