What is the relationship between the couplet at the end and the three quatrains of Sonnet 130?

The three quatrains set the poem up as a way to be more realistic in the narrator's comparisons about his mistress. The rhyming couplet at the end, then turns the sonnet around to really praise his mistress. He says that even though her "eyes are nothing like the sun" (as is the format of the three quatrains), she is "far more rare" than any other poet's mistress who is falsely compared to things like the sun.