Aaron and his brethren arrive in the land of Nephi and present themselves before the king over all of the land. We do not know whether or not they used the technique of volunteering to be servants in the other cities where they had preached, but it appears that they did not. In this case, they have subsequently spoken with Ammon, and have certainly heard of the extraordinary experience Ammon had with Lamoni. It is not surprising, then, that they begin with the same basic premise of presenting themselves to the king as servants.
The difference in this case is that the father of Lamoni has had more preparation for these missionaries than did his son for Ammon. The father of Lamoni has had his owns experience with Ammon, and has seen the determination of his son, a determination to follow the new beliefs that was so strong that Lamoni was willing to commit possible political suicide by rebelling against his over-king. All of these events have, at the minimum, piqued the curiosity of this great king whose name is not recorded.