So each time entry in the invoice is rounded to the nearest hundredth. The problem is that the invoice total is not a summation of these rounded entries. Instead, the invoice total is rounding AFTER summing all time entries.
So half the time the invoice total matches and half the time it does not. It just depends how many time entries were rounded up and how many were rounded down.

To fix this invoices will need to either:
1. Have the invoice total be a total of the rounded entries. (probably preferred but more complicated to program)
2. Round all entries up or down and not the nearest hundredth place.

Just a note that this same issue naturally causes problems when matching exported time entries to the invoice total. So any spreadsheet calculations will not match the invoice total.