The labor hours multiplied by the hourly rate can end up with labor and overhead cost out to 4 decimal places.  The GL posting can only handle 2.  Since the labor posting consists of a single entry on one side (debit to WIP) and potentially 3 separate entries on the other side (labor and overhead) there can be rounding errors.  1.0049 and 1.0049 when rounded to 2 places (as in the GL) and added come to 2.00 but when added as 4 place figures it comes to 1.0098 which then rounds to 2.01.  The program needs the GL postings to balance so the variable overhead posting was arbitrarily selected as theplace to put any odd entries that are a result of these sort of rounding discrepancies.  As you note, they are pennies.