dataFilter function being called twice on collection.fetch()

Given the code below can anyone tell me why the filterFunction is being called twice? It is called twice every time fetch() is called. Is this normal behavior?

locations.xml

<Alloy>
    <Window id="win" title="Buy Locations" class="container" layout="vertical" 
            fullscreen="true" navBarHidden="true" barColor="black">
        <SearchBar id="searchBar" barColor="#7A7979" showCancel="true" 
            hintText="search" height="44" top="0" width="Ti.UI.FILL"/>
        <ListView id="list" defaultItemTemplate="location">
            <Templates>
                <ItemTemplate name="location" height="70">
                    <View height="70" width="50" left="0">
                        <Label class="mapIcon"/>
                    </View>
                    <View left="50" top="0" layout="vertical" width="Ti.UI.FILL">
                        <Label bindId="name" class="name" left="0" top="5" 
                            height="30" width="Ti.UI.FILL"/>
                        <Label bindId="address" class="address" left="0" top="0" 
                            height="20" width="280"/>
                    </View>
                </ItemTemplate>
            </Templates>
 
            <ListSection id="section" dataCollection="retailers" 
                    dataTransform="doTransform" dataFilter="filterFunction">
                <ListItem template="{template}" name:text="{NAME}" 
                    address:text="{STREET_ADDRESS}"/>
            </ListSection>
        </ListView>
    </Window>
</Alloy>

var args = arguments[0] || {};
var brewerID = args.brewerID || '';
var filterCriteria = "";
 
var retailers = Alloy.Collections.retailers; 
retailers.fetch();
 
function doTransform(model) {
    var o = model.toJSON();
    o.template = "location";
    o.STREET_ADDRESS = o.STREET_NUM + " " + o.STREET + ", " + o.TOWN + ", " + 
        o.STATE + " " + o.ZIP;
    return o;
}
 
// filter collection
function filterFunction(c) {
    Ti.API.info("===> In filterFunction");
    var collection = c.where({ID_BREWER: brewerID});
    if(filterCriteria.length > 0) {
        Ti.API.info("filtering collection by:" + filterCriteria);
    }
    return collection;
}
 
$.searchBar.addEventListener("change", function(e) {
    var val = e.value;
    Ti.API.info("searchBar value: " + val);
 
    filterCriteria = val;
    retailers.fetch();
});
 
$.searchBar.addEventListener("cancel", function(e) {
    $.searchBar.value = "";
    $.searchBar.blur();
});