The MIP solver xxx gives an answer different to MIP solver yyy for my problem. Why does this happen ?

The default value of the relative optimality tolerance (optcr)is 0.1. This means that, unless otherwise specified, the solver will stop when it finds a feasible integer solution within 10% of the global optimum. Various solvers can find different solutions that fall in this 10% gap. The user can override this and force the solver to find the global optimum by setting the optcr option to 0.0 using the statement,

  option optcr = 0.0 ;

before the solve statement.