So the underlying thing seems to be that one of your attributes is of type string.

Its most likely the ones which you added latest and soon after started noticing the concatenation.
So, I would suggest reviewing the the types of the attributes.

Here is something more to make it easier to follow:

Below is a script that runs on
-  two Integer attributes (firsthours_int, secondhour_int) and
-  a string attribute (thirdhour_str). 

As long as I use the values read from Integer attributes, the return is an Integer. 
As soon as I start using the "thirdhour_str", the return is a concatenated one.

dojo.provide("com.example.sumitup");

(function() {
    dojo.declare("com.example.sumitup", null, {

        getValue: function(attribute, workItem, configuration) {

var fh = workItem.getValue("firsthours_int");
var sh = workItem.getValue("secondhour_int");
var th = workItem.getValue("thirdhour_str");

var sum = fh + sh + th;  //returns a concatenated of (sum of fh and sh) and th
var sum = fh + sh;  // returns an integer

return sum;

        }
    });
})();

Like Ralph already pointed out, your solution is in using parseInt() when working with strings. 
However, I believe instead of Integer attributes, you have gotten a string typed attribute which is causing the failure for you.

Hope this helps.