Not sure if this counts as Fanon or Speculation, so I'm putting this here:
- It could be possible that Tardy Elves are brought in when a protracted campaign is expected, as opposed to a single battle. Once the fighting begins in earnest, the main force would be not be moving very far between battles. Thus the Tardy Elves will arrive later in the campaign, providing a fresh reserve force to bolster the main army near the conclusion. It is possible that Tardy Elves have low move, which is compensated by other benefits, such as high combat or natural magic.