为什么在此查询中不使用聚簇索引？

I have the following table:

我有下表:

CREATE TABLE [Cache].[Marker](
    [ID] [int] NOT NULL,
    [SubID] [varchar](15) NOT NULL,
    [ReadTime] [datetime] NOT NULL,
    [EquipmentID] [varchar](25) NULL,
    [Sequence] [int] NULL
) ON [PRIMARY]
CREATE TA