Increasing mathematical uncertainty from initial starting conditions is the main reason. But, some of it might be due to the fact that while some of the models share common code, they don’t produce the same results with that code owing to differences in the way CPU’s, operating systems, and compilers work.
G. Krempl, D. Kottke, and M. Spiliopoulou. Proceedings of the 21st European Conference on Artificial Intelligence (ECAI2014), August 18 -- 22, 2014, Prague, Czech Republic, volume 263 of Frontiers in Artificial Intelligence and Applications, IOS Press, (2014)
G. Krempl, D. Kottke, and M. Spiliopoulou. Proceedings of the 21st European Conference on Artificial Intelligence (ECAI2014), August 18 -- 22, 2014, Prague, Czech Republic, volume 263 of Frontiers in Artificial Intelligence and Applications, IOS Press, (2014)