Lady Macbeth's method of greeting of Duncan reflects the advice she had given earlier to her husband. As she states at the end of act 1, scene 5:
Look like th' innocent flower,
But be the serpent under it.
Thus, when Duncan arrives at the castle in act 1, scene 6, Lady Macbeth plays the role of a loyal subject welcoming her king and offering hospitality to her guests. As for Duncan himself, he has no suspicion of any ulterior motives (nor does he necessarily have any cause for such suspicion). It is this tension that creates dramatic irony in this scene: we, the readers, know of Lady Macbeth's true intentions, even as Duncan remains blind to her deception.
This treatment sets the stage for the next scene, when Macbeth is set to carry out the murder (and has a crisis of conscience concerning it). Ultimately, note that Duncan's murder is carried out after he has gone to sleep (when he was in a point of vulnerability). Thus, this deception, which we see present in act 1, scene 6, extends well beyond that initial welcoming of Duncan into the castle. Macbeth and Lady Macbeth continue to present themselves as loyal subjects and gracious hosts, and through this deception, they are able to murder the king and usurp the throne. This murder and usurpation will shape the remainder of the play.