SQL Server 2012 :: Adding RowID To Existing Table - Inconsistent Results

I am getting inconsistent results when BULK INSERTING data from a tab-delimited text file. As part of my testing, I run the same code on the same file again and again, and I get different results every time! I get this on SQL 2005 and SQL 2012 R2.

We have an application that imports data from a spreadsheet. The sheet contains section headers with account numbers and detail rows with transactions by date:

AAAA.1234 /* (account number)*/
1/1/2015 $150 First Transaction
1/3/2015 $24.233 Second Transaction
BBBB.5678
1/1/2015 $350 Third Transaction
1/3/2015 $24.233 Fourth Transaction

My Import program saves this spreadsheet at tab-delimited text, then I use BULK INSERT to bring the data into a generic table full of varchar(255) fields. There are about 90,000 rows in each day's data; after the BULK INSERT about half of them are removed for various reasons.

Next I add a RowID column to the table with the IDENTITY (1,1) property. This gives my raw data unique row numbers.

I then run a routine that converts and copies those records into another holding table that's a copy of the final destination table. That routine parses though the data, assigning the account number in the section header to each detail row. It ends up looking like this:

AAAA.1234 1/1/2015 $150 First Purchase
AAAA.1234 1/3/2015 $24.233 Second Purchase
BBBB.5678 1/1/2015 $350 Third Purchase
BBBB.5678 1/3/2015 $24.233 Fourth Purchase

My technique: I use a cursor to get the starting RowID for each Account Number: I then use the upper and lower RowIDs to do an INSERT into the final table. The query looks like this:

SELECT RowID, SUBSTRING(RowHeader, 6,4) + '.UBC1' AS AccountNumber
FROM GenericTable
WHERE RowHeader LIKE '____.____%'

Results look like this:

But every time I run the routine, I get different numbers!

Needless to say, my results are not accurate. I get inconsistent results EVERY TIME. Here is my code, with table, field and account names changed for business confidentiality.

TRUNCATE TABLE GenericImportTable;
ALTER TABLE GenericImportTable DROP COLUMN RowID;
BULK INSERT GenericImportTable FROM 'SERVERGeneralAppnameDataFile.2015.05.04.tab.txt'
WITH (FIELDTERMINATOR = ' ', ROWTERMINATOR = '', FIRSTROW = 6)
ALTER TABLE GenericImportTable ADD RowID int IDENTITY(1,1) NOT NULL
SELECT RowID, SUBSTRING(RowHeader, 6,4) + '.UBC1' AS AccountNumber
FROM GenericImportTable
WHERE RowHeader LIKE '____.____%'