请教各位老师，vnpy中vnpy.trader.rqdata.RqdataClient 从rq查询数据之后，bar时间特意做了如下调整

INTERVAL_ADJUSTMENT_MAP = {
    Interval.MINUTE: timedelta(minutes=1),
    Interval.HOUR: timedelta(hours=1),
    Interval.DAILY: timedelta()         # no need to adjust for daily bar
}

adjustment = INTERVAL_ADJUSTMENT_MAP[interval]

dt = row.name.to_pydatetime() - adjustment

请问一下，为什么要这么调整？多谢