Yeah, gonna say 'loose cable' as well. You can test it yourself- pedal the bike and pull the triggers or twist the grips into what I'm guessing will be 1/1- your smallest gears. Then reach under the downtube of your bike and pluck the cable on the left side- you'll probably find it to be a little on the floppy side.
Its ok, it happens with newly-built bikes. Your break-in tuneup should fix it.