Tracey,

I think you will need to post DDL and data for us to help you.Otherwise the short answer is "Because they don't match so are not distinct"

Could you build a CTE that includes a RANK() OVER(Partition by Rider, Date ORDER BY TripID )

You can then join the CTE to itself  

SELECT
    CTE1.*
    ,CTE2.*
FROM
   CTE CTE1
LEFT JOIN
    CTE CTE2 ON CTE2.Rider = CTE1.Rider
    AND CTE2.Date = CTE1.Date
    AND CTE2.Rank > CTE1.Rank
    AND CTE2.Pickup = CTE1.Dropoff
    AND CTE2.Dropoff = CTE1.Pickup
ORDER BY
   CTE1.Rider
    ,CTE1.Date
    ,CTE1.Rank


This will give you all of the trips and the return trip if it exists

You will get multiple joins if the rider can do the same round trip more than once in a day but this should at least indicate where the round trips are and why they are not displaying correctly in your distinct