That isn't the bug. You can override the $.fn.calendar.defaults.getWeekNumber function to achieve your own logic.
$.extend($.fn.calendar.defaults, {
getWeekNumber: function(date){
var checkDate = new Date(date.getTime());
checkDate.setDate(checkDate.getDate() + 4 - (checkDate.getDay() || 7));
var time = checkDate.getTime();
checkDate.setMonth(0);
checkDate.setDate(1);
return Math.floor(Math.round((time - checkDate) / 86400000) / 7) + 1;
}
});