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percentage discount not calculating properly

Greetings gurus and list members...
One of my clients has recently noticed a slight peculiarity in the
percentage discount, and I have been able to verify it.
For example, sell 252 items at $1.05 with a 15% discount.  By my handy
dandy calculator application, 252*1.05*.85=224.91.  However, on the
invoice, the total calculated is $224.28.  It is a small difference,
however, it grows when more items are sold.  When we put on another
invoice 784 of the same item being sold at $1.05 with the same 15%
discount, 784*1.05*.85=699.72, however the invoice states under the
extended column that the value should be $697.76, which is a discrepancy
now measuring in full dollars.  
Conversely, selling 7 items at $100 with the same 15% discount, 7*
$100*.85=$595, and it calculates this properly.  But, if I sell those
items at $100.05, it takes things out of whack again:  7*$100.05*.85=
$595.2975($595.30), however the invoice calculates $595.28.  Again, this
is only 2 cents, but that is obviously going to be a problem over a
period of time.
My first suspicion is that this is a rounding issue, but the difference
doesn't work out to be a single penny per item sold.  It also occurred
to me that it may be applying the discount before or after something
else is being calculated on that number, such as tax or another
discount, but again I don't make the numbers add up to anything that
supports that theory.  
Anybody have any theories on what might be causing this?  Or one better,
how to fix it?  Any comments are much appreciated...

Bob Miller
Network, Internet, Server,
and Open Source Solutions