Can the deviation of observed values from expected values be attributed to random chance

Sometimes.

If the deviation has the statistics of the postulated random occurrences, perhaps random chance is involved. If there is systematic deviation, or if the deviation statistics are not consistent with a random process, likely there are causes that are not accounted for.

Very often, the deviation of observed values from expected values indicates an error in the model predicting the expected values. Many scientific advances are made by investigating such deviations.