Hello Armsys,
Maybe the following extract from answer.com would help explain the usage of 'yet':
"
YET
adv.- At this time; for the present: isn't ready yet.
- Up to a specified time; thus far: The end had not yet come.
- At a future time; eventually: may yet change his mind.
- Besides; in addition: returned for yet another helping.
- Still more; even: a yet sadder tale.
- Nevertheless: young yet wise.
conj. And despite this; nevertheless: She said she would be late, yet she arrived on time.
idiom:as yet
- Up to the present time; up to now.
[Middle English, from Old English gīet.]
USAGE NOTE In formal writing, yet
in the sense “up to now” is normally used with an accompanying verb in
the present perfect rather than in the simple past. Thus, one would say
He hasn't started yet, not He didn't start yet. The use of yet with the simple past is common in speech and may be appropriate for informal writing. "
Cheers,
Hoa Thai