Lynn's spot on with this.

Because the Name field is missing from the non-clustered index it would require a seek on the non-clustered followd by a bookmark lookup to the clustered index for the fields missing from the index. That's more expensive than using the Clustered index once to perform the entire query.

If you want to use the non-clustered index, INCLUDE the name field in the index.

i.e.

DROP INDEX UN_MonthlyCMS.[IDX_MonthlyCMS_ProcessYear_ProcessMonth_VisionAccountNumber_CreditPlanNumber]

CREATE INDEX [IDX_MonthlyCMS_ProcessYear_ProcessMonth_VisionAccountNumber_CreditPlanNumber] ON UN_MonthlyCMS(ProcessYear, ProcessMonth, VisionAccountNumber) INCLUDE (CustomerName)