Aura Storage - keeps data for a few seconds? (should keep for a long time)

I'm trying to use Aura storage facility to store some graph data on a client side. to create a storage i use:

function getStorage() {
            var storageName = "GraphsData";

            var storage = $A.storageService.getStorage(storageName);

            if (!storage) {
                console.log("creating storage...");

                storage = $A.storageService.initStorage({
                        name: storageName, // name
                        persistent: true, // persistent
                        secure: true, // secure
                        maxSize: 1024*1024*10, // maxSize in bytes
                        defaultExpiration : 60000, // defaultExpiration (seconds)
                        defaultAutoRefreshInterval: 60000, // defaultAutoRefreshInterval (seconds)
                        debugLoggingEnabled: true, // debugLoggingEnabled
                        clearStorageOnInit: false, // clearStorageOnInit
                        version: "1.0" // version
                    });
            }
            console.log("storage: " + storage);

            return storage;
        }

And it works, i can store some data there by executing:

storage.set("graphData", data);

And retrieve data by:

getGraphData : function() {
        var storage = getStorage();
            return storage.get("graphData").then( function(data) { 
                if (data) {
                    var promise = new Promise(function(resolve, reject) {
                        resolve(data);
                    });
                    return promise;
                }
                else
                {
                     .....

But after 8-10 seconds, when i try to retrieve data from storage again, storage returns undefined (like the data was removed). But why so fast? i played with different initialization parameters, but it changed nothing.

ok, i found that documentation is misleading (wrong).

by looking into component (thru debug console) i found different properties:

AuraStorage {$adapter$: MemoryAdapter, name: "GraphsData", $maxSize$: 10485760, $expiration$: 10000, $autoRefreshInterval$: 30000…}
$adapter$:    MemoryAdapter
$autoRefreshInterval$    :    30000
$debugLogging$    :    false    
$expiration$    :    10000   
$getOperationsInFlight$    :    0
$keyPrefix$    :    "1.0:"
$lastSweepTime$    :    1468848393250
$maxSize$    :    10485760
$sweepInterval$    :    300000
$sweepPromise$    :    undefined
$sweepingSuspended$    :    false
adapter    :    MemoryAdapter
name    :    "GraphsData"
version    :    "1.0"

after setting expiration parameter (instead of defaultExpiration), everything started to work as expected. Now i use

expiration : 600,

for 10 minutes expiration