There's a quote from one of the study authors:

  "Because previous estimation methods relied on coarse five-year snapshots, 
   they yielded very few data points and created the impression that the rate 
   of global migration flows was stable," adds co-author Guy Abel, a research 
   scholar in the Migration and Sustainable Development Research Group of the 
   IIASA Population and Just Societies Program and professor at the University 
   of Hong Kong. "Our annual data provides a clearer picture, revealing that 
   this rate has actually risen since 2000. This upward trend appears to be 
   driven by long-term demographic shifts and economic development rather than 
   sudden, isolated crises."

So if I'm following correctly, when you look at coarse data, you miss a lot of the smaller-scale migration, and that small-scale migration pushes the totals up a lot?