Priorities in Bitcoin-qt Client
The relevant source is this:

if (AllowFree(dPriority)) // at least medium
{
    if      (AllowFree(dPriority / 1000000))  return tr("highest");
    else if (AllowFree(dPriority / 100000))   return tr("higher");
    else if (AllowFree(dPriority / 10000))    return tr("high");
    else if (AllowFree(dPriority / 1000))     return tr("medium-high");
    else                                      return tr("medium");
}


The label only changes to something higher than "medium" when the priority is at least 1000 times larger than the minimum. Your priority is large (I estimate 570/144 * 27 = 107 times the minimum), but it's not that large.