Why Airflow PubSubPullOperator didn't pull max messages?

Question

I use PubSubPullOperator in airflow to pull messages from gcp subscription.

pull_messages_task = PubSubPullOperator(
        task_id="pull_messages",
        ack_messages=True,
        project_id=GCP_PROJECT_ID,
        subscription="k8s-sub",
        gcp_conn_id=GCP_CONN_ID,
        max_messages=50
    )

It works fine to pull messages from subscription and keep in Xcom. My question is why PubSubPullOperator can't pull messages count equal to max_messages every time?

For example, I publish 250 messages to GCP topic. My Dag is run every minutes and pull 50 messages every time.

Below is process log from airflow:

[2022-05-17 14:53:04,630] {pubsub.py:536} INFO - Pulling max 50 messages from subscription (path) projects/production-1/subscriptions/k8s-sub
[2022-05-17 14:53:06,661] {pubsub.py:550} INFO - Pulled 50 messages from subscription (path) projects/production-1/subscriptions/k8s-sub

[2022-05-17 14:54:04,312] {pubsub.py:536} INFO - Pulling max 50 messages from subscription (path) projects/production-1/subscriptions/k8s-sub
[2022-05-17 14:54:06,239] {pubsub.py:550} INFO - Pulled 16 messages from subscription (path) projects/production-1/subscriptions/k8s-sub

[2022-05-17 14:55:04,055] {pubsub.py:536} INFO - Pulling max 50 messages from subscription (path) projects/production-1/subscriptions/k8s-sub
[2022-05-17 14:55:05,259] {pubsub.py:550} INFO - Pulled 4 messages from subscription (path) projects/production-1/subscriptions/k8s-sub

[2022-05-17 14:56:04,590] {pubsub.py:536} INFO - Pulling max 50 messages from subscription (path) projects/production-1/subscriptions/k8s-sub
[2022-05-17 14:56:06,527] {pubsub.py:550} INFO - Pulled 20 messages from subscription (path) projects/production-1/subscriptions/k8s-sub

[2022-05-17 14:57:04,083] {pubsub.py:536} INFO - Pulling max 50 messages from subscription (path) projects/production-1/subscriptions/k8s-sub
[2022-05-17 14:57:07,428] {pubsub.py:550} INFO - Pulled 38 messages from subscription (path) projects/production-1/subscriptions/k8s-sub

[2022-05-17 14:58:05,561] {pubsub.py:536} INFO - Pulling max 50 messages from subscription (path) projects/production-1/subscriptions/k8s-sub
[2022-05-17 14:58:07,431] {pubsub.py:550} INFO - Pulled 50 messages from subscription (path) projects/production-1/subscriptions/k8s-sub

[2022-05-17 14:59:04,348] {pubsub.py:536} INFO - Pulling max 50 messages from subscription (path) projects/production-1/subscriptions/k8s-sub
[2022-05-17 14:59:05,462] {pubsub.py:550} INFO - Pulled 50 messages from subscription (path) projects/production-1/subscriptions/k8s-sub

[2022-05-17 15:00:06,882] {pubsub.py:536} INFO - Pulling max 50 messages from subscription (path) projects/production-1/subscriptions/k8s-sub
[2022-05-17 15:00:08,710] {pubsub.py:550} INFO - Pulled 2 messages from subscription (path) projects/production-1/subscriptions/k8s-sub

[2022-05-17 15:01:03,519] {pubsub.py:536} INFO - Pulling max 50 messages from subscription (path) projects/production-1/subscriptions/k8s-sub
[2022-05-17 15:01:03,688] {pubsub.py:550} INFO - Pulled 20 messages from subscription (path) projects/production-1/subscriptions/k8s-sub

I'm pretty sure that each dag run time is under 1 minute. And 50 messages size is not over Xcom limit(48KB).

Does anyone has any idea about this situation? Or anyone knows how Operator decide how many messages to pull?

Many thanks.