The important sentence prior to "Unlike damage inflicted to standard NPCs (see page 331), damage inflicted to items does not linger." is "When an itemís Damage save is successful, the item may suffer scratches, dings, and other cosmetic flaws, but the attack has no mechanical effect."
So when the Damage Save is successful none of the damage lingers for future hits.
For example, let's contrast an NPC (Damage Save +5) and an object, in this case a main battle tank (Damage Save 8/+35).
You are firing a RSA SVU (1d8+2 damage, no AP).
Your first shot hits and does 7 damage.
NPC makes a damage save (+5) at DC 13 and lets say he make it.
Main battle tanks makes a damage save (+35) at DC 13 and of course makes it.
Your second shot hits and does 10 damage.
NPC makes a damage save (+5) at DC 18 (the 7 initial damage and 10 subsequent damage both apply)..
Main battle tanks makes a damage save (+35) at DC 15 (after succeeding on the previous roll, the tank, like all objects, does not keep lingering damage) and of course makes it.
As you can see from the above, NPCs can accumulate damage, but that puny rifle is never going to be able to bring down the main battle tank as it needs to force a failed damage save in a single hit which isn't going to happen.
Ok, so lets bring down the main battle tank.
Let's hit it with a High Explosive Cruise Missile just for fun (8d10 damage, AP 40,BLST).
Let say the damage is average at 44. The AP 40 reduces the tanks Damage Save to -5. Assuming an average Damage Save of 5 (10 on the die + a -5 save) the tank will fail 4 Damage Saves from the strike and be only a point off a 5th fail.
Since the tank can fail 8 saves, it would still be functional, though a second hit would likely bring it down. If the attack managed to get a critical hit (which would cost 2 dice to activate since the target is huge) then the failed damage saves is doubled, which could break the tank in a single hit.