It happens because you deleted the record. The system won't reset because you deleted the record. Why is it important to have the numbers be continuous? It's possible to use a script that deletes the record and also resets the next serial value of the field, but it also adds a great number of other issues to the design/function of your database and only works if you deleted the most recently created record--which can be difficult and impossible to know if you have several users that might all be adding new records at the same time.
If you must have a continuous series of serial numbers--such as to keep auditors happy with your data, it's better not to delete any records. Instead mark them "void". Then the record is there to account for the serial number, but you use the record's "void" status to keep it's data out of reports and such.