flinksql写kafka，有时输出数据连续，有时却不连续？？

flinksql写kafka，开启了Checkpoint，并且设置了CheckpointingMode.EXACTLY_ONCE，写入kafka时没有设置kafka事务，为啥有时出现连续写入数据到kafka（如图1），有时又是周期性写入kafka（如图2）（周期性为Checkpoint的时间间隔）？

图1

图2

sink kafka 表是：

create table dwd_spyw_alert_info_stat_kaf_sk(
comb_type string
,period_type string
,user_id STRING
,alert_type STRING
,region_id STRING
,device_code STRING
,shop_id STRING
,alert_time STRING
,insert_time STRING
,primary key(comb_type, period_type, device_code, user_id, region_id, alert_type, shop_id) not enforced
)with(
'connector' = 'upsert-kafka',
'topic' = 'topic-name',
'properties.bootstrap.servers' = 'localhost:9092',
'key.json.ignore-parse-errors' = 'true',
'value.json.fail-on-missing-field' = 'false',
'value.json.ignore-parse-errors' = 'true',
'key.format' = 'json',
'value.format' = 'json',
'sink.buffer-flush.max-rows' = '5000',
'sink.buffer-flush.interval' = '1 s',
'properties.compression.type' = 'gzip',
'properties.compression.codec' = 'gzip',
'properties.linger.ms' = '5'
);