Killzone 3 was more undertracked because NPD does't track units sold with hardware bundles while VGC does.
actually it doesn't look like it.
The Source wrote PS3 sold 328k this month, which is 87% of the Americas sales
He also wrote Killzone 3 sold 205k, which is only 81% of the Americas sales (87% would be 221k)
so unless he uses a different percentage specifically for PS3 games (which I didn't check and I doubt since he uses the same one for the PS3 consoles) he either made a mistake or took off 16k that were sold as bundles, thus making it undertracked by 75k.
And 87% is the usual marketshare of the US in the Americas.