Quantity invoiced calculation

How quantity invoiced is calculated .

I know the Formula ,

If  Amount and Unit price are present .

System, calculates  it as 

Amount/Unit Price =Quantity invoiced in ap .

How  it rounds Of   If Quantity Coming in Fraction

amount :10

Unit price .5

For Ex  10/3.5  =2.857142857142857.

But the Quantity invoice comes in Differently in Ap.

Can  any body explain what precision is takes while Rounding OFF.

IS it like   Round(2.857142857142857,15)  =2.85714285714286