Hi dropoffyourkeelee,
I think there is more to it, than that. Here is your table, with the number of baptised added as an extra column:
2010 151,842 10,711
2011 161,298 6.2% 11,593
2012 163,349 1.3% 10,668
2013 174,687 6.9% 19,501
2014 180,343 3.2% 11,052
2015 176,585 -2.1% 10,274
Over the same period, population increased by 13% but publisher numbers increased by 16%.
If the figures are to be believed, what they seem to show is that over the past few years, they are very good at baptizing people, but poor at retaining them. Last year, baptisms were a little down and retention was even worse. If the figures were fudged, they were probably fudged every year.
My bet is that it is a retention issue.
Also, for logistical and geographical reasons, I don't think a significant number could have gone across a border, to (for example) Angola. It is a very big country in both area and population. May be a few, but not 10% or more.