Javascript changing cue startTime sets wrong value

Question

I`m trying to change startTime of cues in html5 video element, but wrong result is written to startTime.

Chrome console output:

>track[0].startTime
<52.152
>track[0].startTime = 60
<60
>track[0].startTime
<54.988

Actual code:

var track = video.textTracks[0].cues;
        if(typeof(track) != "undefined")
        {
            for(var i = 0; i< track.length; i++)
            {
                console.log("Start time before offset: "+track[i].startTime);
                track[i].startTime += 10;
                console.log("Start time after offset: "+track[i].startTime);
            }
        }

I can`t figure out, why is such a strange result produced.