Why the Serial No. does match the Date for the released ACT Real Test?

Some of the sequence and the date of the ACT Real Test Sample made me confused.
Since I found 61F is in 2004 but 61B is in 2008. Shouldn’t 61B be put before 61F since B is before F in the alphabetic sequence, however, 2008 is after 2004. Why the sequence is like that?
Anyone please enlighten me!
Thx!