Nonclustered index is faster than clustered index?
Yes the clustered index has fewer rows per page than the non clustered index as the leaf pages of the clustered index must store the values for the other two columns (FirstName and LastName).

The leaf pages of the NCI store only the BusinessEntityId values and a row locator (RID if the table is a heap or the CI key otherwise).

So the estimated costs reflect the greater number of reads and IO requirement.

If you were to declare the NCI as

nonclustered index on Persontb_NC(BusinessEntityId) INCLUDE (FirstName, LastName)


then it would be similar to the clustered index.