One excuse I recall hearing is that sometimes the absentees are not counted because the number is sufficiently small that they could not possibly affect the outcome of any election.
Complete urban legend. Sometimes the MEDIA stop reporting results before the absentees are counted, but every office counts the votes (albeit sometimes sloppily) and reports them in the final certified numbers. This circulated in 2000 (and probably before), and several Sec State sites put out statements contradicting it.