In "Ode on a Grecian Urn," Keats spends most of the poem describing what he sees on the Grecian urn in the title. He shows us various scenes that appear on the urn, exploring them from his own perspective as someone who is meditating on them centuries after the urn itself was created, as well as centuries after scenes like those on the urn might have taken place.

Toward the end of the poem, however, Keats begins to turn away from just describing the urn. Instead, he starts to think about what the urn might mean—that is, what its existence in Keats's "present" might communicate.

As long as he sticks to just description, Keats can be pretty confident about what he's saying. After all, it's right in front of him. When Keats describes "pipes and timbrels" or a chase between a young man and a maiden, he has a reference point in reality. He can say those things are on the urn because he can see them.

Once Keats starts to think about what the urn might mean, however, he's not as confident. Now, he's aware that he's not just describing the urn. Instead, he's adding his own interpretations.

The more Keats moves away from describing the urn's scenes and into exploring their meaning, the more the rhyme scheme begins to vary.

In the seventh line from the end, Keats says that the urn "doth tease us out of thought" entirely. The last six lines may show a different rhyme scheme from the rest in order to emphasize that we are now "out of thought." Maybe we're dreaming, maybe we're feeling, maybe we're imagining; but we're not simply thinking and describing the way the earlier stanzas of the poem did. As this orderly type of approach breaks down, so does the rhyme scheme that communicated it.