Charles Ellicott Commentary Exodus 16:18

"And when they measured it with an omer, he that gathered much had nothing over, and he that gathered little had no lack; they gathered every man according to his eating." — Exodus 16:18 (ASV)

When they did mete it with an omer. —Each Israelite gathered what he supposed would be about an omer for each member of his family. Some naturally made an overestimate, some an underestimate; but whatever the quantity collected, when it came to be measured in the camp, the result was always the same—there was found to be just an omer for each. This result can only have been miraculous.