calendar widget strange behaviour

i was wandering why calendar widget does not enclose calendar itself into
first

• as happens for all the other widgets,

this disrupts css…

<li><h2>Calendar</h2></li><li><table id="wp-calendar" summary="Calendario">........

strangely on the other themes previously installed with identical sidebar.php the result is-:

<li id="calendar" class="widget widget_calendar">Calendario<div id="calendar_wrap"><table id="wp-calendar" summary="Calendario">
……………

what this depends on?