$q.all returns same response for all promises in the array

I'm facing some difficulties while trying to understand why $q.all returns the response of the last promise in array, for all promises given:

function getGlobalData() {
    $q.all(
        [
        genericApi.submitPostRequest(getPostData('firstKey')),
        genericApi.submitPostRequest(getPostData('secondKey')),
        genericApi.submitPostRequest(getPostData('specialKey'))
        ])
    .then(function(results){
        vm.globalObject['firstKey'] = results[0].data;
        vm.globalObject['secondKey'] = results[1].data;
        vm.globalObject['specialKey'] = results[2].data;
    });
}

End-points are all the same, the only thing I change on each request is one element (key element) in 'postData' object.

function submitPostRequest(data) {    
    return $http({
        method: 'POST',
        data: data,
        url: 'https://someUrl',
        headers: {
            'Content-Type': 'application/json',
            Authorization: 'Bearer someToken'
        }
    });
}

postData :

var postRequest = {
  'endtime' : null,
  'key' : null, 
  'arr' : ['diff','core'],
  'starttime' : null
};

getPostData:

function getPostData(key){
    postRequest.key = key;
    return postRequest;
}

Use angular.copy to make a new copy of the data for each request:

function getPostData(key){
    var req = angular.copy(postRequest);
    req.key = key;
    return req;
}