[R] calculate differences - strange outcome

[Kunzler, Andreas]

Dear List,

I ran into some trouble by calculating differences. For me it is
important that differences are either 0 or not.

So I don't understand the outcome of this calculation

865.56-(782.86+0+63.85+18.85+0)
[1] -1.136868e-13

I run R version 2.71 on WinXP

I could solve my problem by using
round()
but I would like to know the reason.
Maybe someone can help me?

Thanx