The national region is all the leftover points that didn't reach the 50 mark in the smaller regions. They get transferred up, and it's only at that region where any rounding occurs. If you didn't reach 50 points, if you got 49 points, it would be transferred to the next region, but in the last region there would have to be some rounding. This is why this system is more proportional than other mixed systems because other mixed systems will round at the first top-up region.
On October 5th, 2016. See this statement in context.