Also, there is no black-and-white definition of "infected".
Let's say a tester scans his system after running a malware sample, and he finds some junk. Maybe it is just dead, inert files that will never hurt anybody. Harmless debris from a foiled attack.
Even if there is an actual malware process running, it might be just stage one that never spawned the payload.
Even if the payload succeeded in running, maybe the firewall blocked it from doing actual damage.
So even if the AV allowed the system to become "infected", it might mean that the malware won the first battle, but in the end, your system won the war.