Charles Ellicott Commentary Numbers 9:20

"And sometimes the cloud was a few days upon the tabernacle; then according to the commandment of Jehovah they remained encamped, and according to the commandment of Jehovah they journeyed." — Numbers 9:20 (ASV)

And so it was ... — Better, And sometimes, etc.; that is, there were times or occasions when this happened, and so on. So in Numbers 9:21.