Almost all (44 of 45) countries are above the 1∶1 line, indicating that the datasets used to calculate buffer growth rates provide consistently higher rural growth rates than does the UN dataset.
http://plosone.org/article/info:doi/10.1371/journal.pone.0004279#pone-0004279-g003

It's also possible to find consistently higher "rural" growth rates in the spatially explicit model if you fail to mask out urban areas. Unfortunately, this is exactly the problem with using the CIESIN GRUMP data, as Joppa et al. have done. Although the urban extents they use do tend to fall on or around population centers, they by no means mask out all urban areas. As a consequence, calculating growth rates in rural areas by this method is, in fact, calculating growth rates in rural areas as well as most urban areas, leading to artificially-inflated rates.