First say your on the dutch speeking forum for FileMaker.
The Serial number is propably given when the record is made.
When you delete the record and then create a new record, you get a gap.
This is normal behaviour for the 'Serial key' option.
I you don't want that, you should use a 'Calculated value, and take always the Max()+1.
Or you should script it.
Good luck with it,