Try moving the your assignment of the 'performances' variable inside your MyIndividual.setup() method.
It's hard to know without seeing your code, but I suspect that every individual in your population is actually storing a reference to the same instance of 'Measures'. This line is only called when the MyIndividual constructor is called:
private Measures performances = new Measures();
But when ECJ creates the initial population, it only uses the default constructor of your Individual once, to initialize a "prototype" instance of the Individual. The population itself is created by cloning this prototype individual and then calling setup() on the clones.
So, to initialize a new Measures object for each clone, you either need to do it in the setup() method, or override MyIndividual.clone() to perform a deep copy. Using setup() is cleaner, though, since it follows ECJ's convention, and you don't actually care about copying the data.