variable scope within angular service

I have a custom service, which has a dependency on another service and has an external api and some internal functions.

angular.module('DashboardApp').factory('layerQueryer', layerQueryer); 

function layerQueryer($http, $q, overlayLayersConf) {
    layerQueryer.$inject = ['$http', '$q', 'overlayLayersConf'];

    function joinArrays(array1, array2, keya, keyb) {
            .....
    };

    function getLayerList() {
        var def = $q.defer();
        var Promise1 = dosomething();
        var Promise2 = dosomethingelse();
        var Promise3 = doanotherthing();
        $q.all([Promise1, Promise2, Promise3])
            .then(function (data) {
                def.resolve('xxxx');
            })
            .
        catch (function (error) {
            console.log(error);
        })
        return def.promise;
    };
    return {
        getLayerList: getLayerList,
    };
}

I have two problems with this service - one when the callback executes after the three promises are fulfilled I no longer have a reference to the overlayLayersConf service passed into the service constructor, which I need, and I also do not have a reference to joinArrays when I need to use it around the same time.

I have found that if I turn the JoinArrays function into a variable and make an additional two functions and expose them:

function getJoinArray(){
    return joinArrays;
}

// for some reason this seems to allow the scope to be kept during the promise callback...
function getOverlayLayers(){
    return overlayLayersConf;
}

return {
    getJoinArray:getJoinArray,
    getLayerList: getLayerList,
    getOverlayLayers: getOverlayLayers,
};

Then I can get references to these variables. This seems intuitive but strange at the same time. Can anyone tell me what is going on here?