An exercise is saved once the student presses the 'Finish' button. On the exercise results page, you can see all exercises a student completed. Every activity in a student account is saved in a log file. Results can't disappear or be removed from the results page or from the log file.
In English Essentials, there is a counter that counts the number of passes and fails for each skill. This counter is reset at the start of the new school year. Older results will not be included in the counter, but you can still see older results on the exercise results page and in the student's timeline.
When there appears to be a discrepancy in an exported results file, this might be due to the fact that the results export counts only one instance of each exercise. If a student completes an exercise more than one time, only one instance of this exercise is counted in the export.