Exchange variance on home currency transaction

I have this strange behavior : some GL post on the exchange variance  account on transactions in the home currency.
Everything is in the home currency (payment and invoice) so I don't really know why we are here using the exchange  variance account.

However, those particular transaction use a discount, and it seems that the amount posted as exchange  variance correspond to the difference between the sum of the rounded discount and rounded sum of the discount.

The current transaction looks like

4000    Sales    1 Summer 2012             9.50    
4000    Sales    1 Summer 2012         2.38        
4000    Sales    1 Summer 2012             9.50    
4000    Sales    1 Summer 2012         2.38        
4000    Sales    1 Summer 2012             9.50    
4000    Sales    1 Summer 2012         2.38        
4000    Sales    1 Summer 2012             9.50    
4000    Sales    1 Summer 2012         2.38        
4000    Sales    1 Summer 2012             9.50    
4000    Sales    1 Summer 2012         2.38        
4000    Sales    1 Summer 2012             9.50    
4000    Sales    1 Summer 2012         2.38        
4000    Sales    1 Summer 2012             9.50    
4000    Sales    1 Summer 2012         2.38        
1100    Debtors Control Account              59.89        
2200    VAT (20.0%)                  9.98    
4920    Foreign Exchange Gain                  0.07    
Total            76.55    76.55    

7 items are sold for 9.50 with 25% discount (2.38 ). On the invoice they appears with a price of 7.13 and the total of the invoice is 59.89 which correspond to the line :  Debtors  Control Account, 59.89

However 9.50 - 2.38 is 7.12 (instead of the 7.13 invoiced) so I guess the FX gain of 0.7 correspond to 7 times this 0.10 difference.

So that's not a big deal, everything is pretty much alright. I'm just wondering why do we need to use this Exchange variange account instead of posting the correct discount : ie
If the price of the item is 9.50 and we apply 25% discount (2.375) resulting in a rounded price of 7.13, shouldn't the discount be

   9.50 - 7.13 (ie price - rounded(price*(1-discount)) 

rather than

   2.38 (rounded(price*discount))

(or as the price is round up the discount should be rounded down ...).

I understand anyway, than it's wise to add at the end a stage to fix any rounded error and post them in a exchange variable account (even though I'm not sure that's the right one) but couldn't amount be right at the first plate ?

So my question is, is this a bug or a know issue which is considered as irrellevant (or does that need a different GL account)

/Elax