Well, the problem is with roundings and VirtueMart.
VirtueMart does not store all the fields for an item - meaning brutto price, tax + netto price. So in some cases, tax or brutto price needs to be calculated from the other two values - and in such cases, this kind of incosistency may occur.
There is a setting in VM Invoice you can use to switch the calculation method. (data from VM - but then some sums on invoice may not fit; or recalculation based on VM data -> but then you can get results like this)