Normally the view is updated from top to bottom. This usually is not a problem, but under some circumstances the values for data is not available until after the full view is computed. If such values are needed before they are computed the order of evaluation can be change using ng:eval-order
<ANY ng:eval-order="priority"> ... </ANY>
priority(optional=0) – {integer|string} –
priority integer, or FIRST, LAST constant
try changing the invoice and see that the Total will lag in evaluation