Why can Excel 2007 not calculate sub formulas right?

I have the simplest possible scenario:

• open a new Excel 2007 document
• In the first row put 6.99
• In the second row put 4.99
• In the third row put =A1*0.4
• And in the fourth row =SUM((A2-A3)/A2)

This will print out 0.439679359 but every calculator, plus taking out the sub formula will retu 0.438877756 which is correct.

Why is Excel incorrect when using a sub formula?