OECD GDP per capita - a question

The NZ Initiative featured this graph (courtesy of Capital Economics) in its weekly newsletter today. It poses a question for me. Why is the Australian line so erratic compared to NZ's? A guess: there is more competition/changeability of rankings amongst the richest nations, whereas relativity between the poorer nations (of which NZ is one) is steadier. (The 10 years to 2011 gdp per capita stats are here. Both countries showed steady growth from 2002 flattening over 2008-11 but Australia's gdp per capita grew by 14 percent whereas NZ's grew by 6.)