You have coded the accumulative value of a field, so basically record 1 + record 2 + record 3 + record 4.......
You should be using the calculate sequential ID or number based on an interval.
I see the following steps:
1) get a listing of unique values within the column ( if there are not too many, you could actually enter these into a list in the expression)
2) for each unique value, if the record == the value, run the sequential ID, appending the unique value to it. So if the input column = A, then it runs the code, if not, it goes to the next record, and if it = A, it runs the code.... then it switches to "B" and does the same sequence.