These evaluations are always integers, so there are 2 possible problems: Math.round is unnecessary and loss of precision. Try this benchmark:

    public static void main(String[] args) {
        int samplesToAdd = 1, curThreads =1 ,  threadCount = 3;
        int value = samplesToAdd * (curThreads)/ threadCount;
        System.out.println("value = " + value);
    }