As rothgar13 says: it is a common mistake. But ranks are not Rank & File models.
BRB p 83 (for Monstrous Cavalry):
A rank of monstrous cavalry needs only three models to count for rank bonus, steadfast and so on, and only need a six model frontage to count as a horde, as described under monstrous infantry (see page 81).
BRB p. 81:
As monstrous infantry are so much larger than normal troops, they require fewer warriors to fill out a rank. Where most troop types need five models for the rank to count towards rank bonus, steadfast and so on, a unit composed purely of monstrous infantry needs only three models. Essentially, wherever the rules say 'a rank of five or more models' treat it as reading 'a rank of three or more models'.
Look Out Sir, however, is about 5 Rank and File models:
BRB p. 99:
If a character is hit by such an attack, and he is in a unit of at least 5 rank-and-file models (excluding the champion, if there is one) and he is of the same troop type as the unit, then his controlling player is permitted to make a "Look Out Sir!" roll to save him from harm. This works exactly as the "Look Out Sir!" rule described for unit champions on page 93 . Simply put, on a roll of 2 or more, the hit does not strike the character, but instead is allocated to a rank-and-file model. If there are less than 5 rank-and-file models, the character does not get a Look Out Sir! roll and is hit.