Hey.
If the serial number column - which I presume is a simple integer field - is marked AUTO_INCREMENT, then MySQL automatically assigns it the next number that has not been used. If you have previously deleted a row, the serial number it used will not be reused.
This - I assume - is partly done to simplify, and thus increase the performance, of the AUTO_INCREMENT number generation, and to maintain the integrity of the data. Meaning: in a MyISAM table, a deleted row could still potentially be referenced by other tables (FK relationships are not enforced), so re-using the ID of that row might create unwanted references between tables.
If you want your code to re-use previously used, and deleted, serial numbers, you will have to write a routine to do that manually. - Granted, I know little of .Net, so there may well be some built in feature that handles this, but I wouldn't know. (Try asking in the .Net forum.)