Differences using gcloud table vs CSV format.

Table format

gcloud compute snapshots list \
--filter="sourceSnapshotSchedulePolicy ~ $POLICY AND sourceDisk~$disk$" \
--format="table(sourceDisk.basename(),name,creationTimestamp,sourceSnapshotSchedulePolicy.basename())" \
--sort-by "~creationTimestamp" \
--project="$PROJECT"

CSV format

gcloud compute snapshots list \
--filter="sourceSnapshotSchedulePolicy ~ $POLICY AND sourceDisk~$disk$" \
--format="csv[separator=','](sourceDisk.basename(),name,creationTimestamp,sourceSnapshotSchedulePolicy.basename())" \
--sort-by "~creationTimestamp" \
--project="$PROJECT"

Output to file

gcloud compute snapshots list \
--filter="sourceSnapshotSchedulePolicy ~ $POLICY AND sourceDisk~$disk$" \
--format="csv[separator=','](sourceDisk.basename(),name,creationTimestamp,sourceSnapshotSchedulePolicy.basename())" \
--sort-by "~creationTimestamp" \
--project="$PROJECT" > output.csv