Regarding processors of computer
 

My computer consists of 12 processors First i ran Fluent simulation by giving 10 number of processors then convergence of mass going upto 10^-5 and when i ran Fluent simulation by giving 4 number of processors convergence of mass going upto 10^-7. I got a doubt why there is difference in convergence by changing number of processors of computer?
YOUR HELP IS MUCH APPRECIATED.