Viking is not in its best form as in the last seven (1-1-5) it has only won against Ham Cam away from home (1-2), while its last "ace" was in July (2-1 against Kristiansund with penalty at 9'). There is also the cup match in three days with Kristiansund, so there is an additional risk of breaking the favorite.
Of course, Lillestrom is not going through the best period of its form either, but it still remains in the bay of the European ticket, therefore it has a good chance for the three points.