为什么在序列化隔离级别事务中MERGE查询会死锁？

MERGE

CREATE PROCEDURE MergeIt
    @dataToMerge MyTableType READONLY
AS
    SET TRANSACTION ISOLATION LEVEL SERIALIZABLE;
    SET XACT_ABORT ON;

    BEGIN TRANSACTION

        MERGE INTO TargetTable WITH(HOLDLOCK) AS [target]
        USING @dataToMerge AS [source]
        ON [source].KeyPart_1 = [target].KeyPart_1 AND
           [source].KeyPart_2 = [target].KeyPart_2
        WHEN NOT MATCHED THEN
            INSERT(Data, KeyPart_1, KeyPart_2)
            VALUES([source].Data, [source].KeyPart_1, [source].KeyPart_2)
        WHEN MATCHED THEN
            UPDATE SET [target].Data = [source].Data,
                       [target].KeyPart_1 = [source].KeyPart_1,
                       [target].KeyPart_2 = [source].KeyPart_2;

    COMMIT TRANSACTION
RETURN 0

TargetTable[KeyPart_1, KeyPart_2]MyTableTypeTargetTable[KeyPart_1, KeyPart_2]

MERGESERIALIZABLE[KeyPart_1, KeyPart_2]UPDATE

TABLOCKXSERIALIZABLE

谢谢！