Array modification while iteration

When trying to modify the current item while iterating over an array the modification fails. Below is the sample code.

var s_arr = [{a: 1, b: 2}, {a: 3, b: 4}, {a: 5, b: 6}];
var arr = [];

for(var i in s_arr) {

  if(s_arr[i].a == 5) {

    s_arr[i].b = 0;
    console.log('First modification: ' +JSON.stringify(s_arr[i]));
    arr.push(s_arr[i]);

    s_arr[i].b = 9;
    console.log('Second modification: ' +JSON.stringify(s_arr[i]));
    arr.push(s_arr[i]);
  }

}

console.log('Final: ' +JSON.stringify(arr));

After running the above script node test.js, below is the result.

First modification: {"a":5,"b":0}
Second modification: {"a":5,"b":9}
Final: [{"a":5,"b":9},{"a":5,"b":9}]

Expected result is as below.

First modification: {"a":5,"b":0}
Second modification: {"a":5,"b":9}
Final: [{"a":5,"b":0},{"a":5,"b":9}]

However when adding new object while iteration & assigning each values of current item (object) extensively works.

var s_arr = [{a: 1, b: 2}, {a: 3, b: 4}, {a: 5, b: 6}];
var arr = [];

for(var i in s_arr) {

  if(s_arr[i].a == 5) {

    s_arr[i].b = 9;
    console.log('Second modification: ' +JSON.stringify(s_arr[i]));
    arr.push(s_arr[i]);

    var a = {};
    a.a = s_arr[i].a;
    a.b = 0;
    arr.push(a);

    var b = {};
    b.a = s_arr[i].a;
    b.b = 9;
    arr.push(b);
  }

}

console.log('Final: ' +JSON.stringify(arr));

Below is the result of modified script.

Final: [{"a":5,"b":0},{"a":5,"b":9}]

Why is the first script when ran shows modification of object right but the final array shows which consists of modified objects is not as expected?